A vaccine is a biological preparation that gives the body active, acquired immunity to a disease. Most vaccines are preventive: they teach the immune system to recognize a germ before a real infection happens. Some newer ones are therapeutic and are designed to help fight a disease, such as a cancer, that is already present. Public health agencies describe vaccination as one of the most effective ways ever found to prevent infectious disease.
Behind every vaccine is a long chain of work. Scientists have to understand the germ, choose a part of it for the immune system to target, find a safe way to present that target, test the product in animals and then in thousands of people, build factories that can make identical batches, win approval from regulators, and then keep watching for rare problems after millions of doses have been given. This article walks through that chain. It starts with the history that created the field, explains the main kinds of vaccines in use today, and then follows a candidate vaccine from the laboratory to the clinic and beyond. It ends with what the COVID-19 pandemic changed and a short FAQ.
Before vaccines: variolation
The idea that surviving a disease protects you from catching it again is very old, and people tried to use it deliberately long before anyone knew what a virus was. The disease they targeted was smallpox, which killed roughly 20 to 30 percent of the people it infected.
The practice, called inoculation or variolation, involved deliberately exposing a healthy person to material from a smallpox patient in the hope of causing a mild case. It was probably practiced in Africa and China before it reached Europe. The first clear written reference comes from the Chinese author Wan Quan, in a book published in 1549. One Chinese method involved blowing powdered smallpox scabs into the nose. Variolation was not safe, since the mortality rate was about 0.5 to 2 percent, but that was far lower than the death rate from catching smallpox naturally.
Lady Mary Wortley Montagu, an English aristocrat who had seen the practice in the Ottoman Empire, helped introduce it to Britain in the 1720s. Variolation became fairly common in the eighteenth century, and it set the stage for the first true vaccine.
Edward Jenner and the first vaccine
In the late eighteenth century, the English doctor Edward Jenner tested a piece of rural folk knowledge: people who had caught cowpox, a mild disease picked up from cattle, seemed to be protected against smallpox. On 14 May 1796 Jenner inoculated James Phipps, an eight-year-old boy, with material from a cowpox sore. He then showed that the boy was protected against smallpox.
Jenner called cowpox variolae vaccinae, "smallpox of the cow," from the Latin vacca, meaning cow, and he published his findings in 1798. The word "vaccine" grew out of that term. The virus used in later smallpox vaccines became known as vaccinia. In 1939 the scientist Allan Watt Downie showed that vaccinia was serologically different from cowpox, and it was later recognized as a separate virus species.
Jenner's approach had a big advantage over variolation. It used a related but milder virus rather than smallpox itself. Using a related animal germ to protect against a human disease is sometimes called a "Jennerian" or heterologous approach. The BCG vaccine against tuberculosis, which is based on a related bacterium, belongs to the same family of ideas.
Smallpox eradication: the proof of concept
Smallpox vaccination spread around the world during the nineteenth and twentieth centuries, but the disease survived in many countries. In 1967 the World Health Organization (WHO) began an intensified global campaign, coordinated by a unit led by the American epidemiologist Donald Henderson. Its strategy combined vaccination with an active search for outbreaks, so that cases could be found and contained quickly.
The last known endemic case of smallpox occurred in Somalia in October 1977. In December 1979 a commission chaired by the Australian virologist Frank Fenner concluded that smallpox had been eradicated, and the World Health Assembly endorsed that conclusion in May 1980. Smallpox is still the only human disease ever eradicated. It showed that a vaccine, used systematically, could remove a disease from the planet.
Louis Pasteur and the idea of attenuation
The next major step came from the French chemist Louis Pasteur in the 1870s and 1880s. Pasteur's insight was that a germ could be deliberately weakened in the laboratory, a process he called "attenuation," and then used as a vaccine. In 1881 he proposed extending the words "vaccine" and "vaccination" to these new protective inoculations, in honor of Jenner.
Pasteur's first vaccine research targeted chicken cholera, a bacterial disease of poultry. Much of the hands-on work was done by his assistants Émile Roux and Charles Chamberland, partly because a stroke in 1868 had limited Pasteur's ability to run experiments himself. Later work showed that the chicken cholera vaccine did not reliably produce immunity.
His most famous public demonstration was an anthrax trial at Pouilly-le-Fort in France in May and June 1881. Fifty-eight sheep, two goats and ten cattle were used, with about half vaccinated. The vaccinated sheep and goats survived when exposed to anthrax, while the unvaccinated animals died or were dying. The history is more complicated than the legend, though. The veterinarian Henry Toussaint had developed an anthrax vaccine in 1880, and Pasteur's notebooks show that the vaccine used in the public trial was killed with the chemical potassium dichromate, a method similar to Toussaint's, even though Pasteur described it as "live."
Pasteur's rabies vaccine, developed largely by Roux, was made by growing the virus in rabbits and weakening it by drying infected nerve tissue. On 6 July 1885 it was given to Joseph Meister, a nine-year-old boy who had been mauled by a rabid dog. Pasteur was not a licensed physician, so the injections were given by the doctor Jacques-Joseph Grancher under his supervision. Meister received 13 inoculations over 11 days and survived. In 1886 Pasteur's group treated 350 people, and only one developed rabies.
These episodes also show that early vaccine work was done with far less testing than would be allowed today. Pasteur said that 50 dogs had been tested before the first human use, but the historian Gerald Geison's study of the notebooks found that only 11 had been vaccinated. The modern system of staged trials, independent review and informed consent grew partly out of lessons like this.
The twentieth century: polio and the rise of modern vaccine science
Vaccine science expanded quickly in the twentieth century. The first approved inactivated influenza vaccines were developed by the U.S. military in the 1940s and used during the Second World War. Toxoid vaccines, made from inactivated bacterial toxins, were developed against tetanus and diphtheria.
Polio became the defining vaccine story of the mid-century. A key advance came in 1948, when John Enders's group at Children's Hospital Boston grew poliovirus in human tissue culture, which made large-scale vaccine research practical. In 1952 and 1953 Jonas Salk's team developed an inactivated, injected polio vaccine (IPV). In 1955 the Francis Field Trial, then the largest medical experiment ever conducted, reported that the Salk vaccine was about 60 to 70 percent effective against poliovirus type 1 and more than 90 percent effective against types 2 and 3.
The same year brought one of the most important safety lessons in vaccine history: the Cutter incident. Vaccine from one manufacturer had not been fully inactivated, and some recipients developed paralytic polio. The episode damaged public confidence and pushed governments toward much stricter oversight of how vaccines are manufactured and tested.
Albert Sabin's live, weakened oral polio vaccine (OPV) entered commercial use in 1961, and a trivalent form covering all three poliovirus types was licensed in 1963. Oral vaccine was cheap and easy to give, and it largely replaced the injected vaccine in many places. In 1988 the WHO, UNICEF and Rotary launched a global polio eradication effort. Reported cases fell from an estimated 350,000 that year to 33 in 2018.
Polio also shows a trade-off of live vaccines. In rare cases the weakened virus in oral vaccine can circulate among under-immunized populations and regain the ability to cause paralysis. After wild type 2 poliovirus was eradicated, the trivalent oral vaccine was replaced in 2016 by a bivalent version without type 2. Cases of circulating vaccine-derived type 2 polio then rose, from 2 in 2016 to 1,037 in 2020, and in 2017 vaccine-derived cases outnumbered wild poliovirus cases for the first time. Scientists responded with a genetically stabilized oral type 2 vaccine, nOPV2, which received emergency licensing in 2021 and full licensure in December 2023. This is a good example of vaccine development continuing long after a product first reaches the market.
How vaccines work: antigens, immunity and memory
All vaccines rely on the same basic biology. The immune system learns to recognize specific molecular features of a germ, called antigens. A vaccine presents those antigens, or instructions for making them, without causing the full disease.
The body's defenses work in two broad layers. The innate immune system responds quickly, often within about 12 hours, and is not specific to one germ. The adaptive immune system is slower, taking one to two weeks to develop a full response, but it is highly specific. It includes B cells, which make antibodies that can latch onto a germ and help neutralize it, and T cells, which help coordinate the response and can destroy infected cells.
The key feature that vaccines exploit is immunological memory. After the first encounter with an antigen, some immune cells become long-lived memory cells. If the real germ appears later, these cells can mount a faster and stronger response. Vaccines do not work equally well in everyone. An estimated 2 to 10 percent of people do not respond well to a given vaccine for host-related reasons, which is one reason why high vaccination coverage in a community matters for protecting people whose own response is weak.
Many vaccines also contain an adjuvant, a substance that strengthens the immune response. Aluminum salts are the most common example; tetanus toxoid, for instance, is usually adsorbed onto alum. A vaccine dose is mostly water, and the active ingredient may amount to only nanograms of virus particles or micrograms of bacterial sugars.
The main types of vaccines
Over two centuries, scientists have developed several different ways to show the immune system an antigen. Each approach has strengths and weaknesses, and the choice affects safety, how strong and long-lasting the protection is, how the vaccine is made and how it must be stored.
Live attenuated vaccines
These use a living germ that has been weakened so that it causes little or no illness. Examples include the vaccines against measles, mumps, rubella and yellow fever, and the oral polio vaccine. Because the weakened germ multiplies a little in the body, these vaccines often produce strong, long-lasting immunity. The downside is that they may not be safe for people with weakened immune systems, and, as with oral polio vaccine, a weakened virus can occasionally change in ways that matter.
Inactivated vaccines
These use a germ that has been killed, usually by chemicals or heat, so it cannot multiply. Examples include the injected polio vaccine, hepatitis A vaccine, rabies vaccine and most influenza vaccines. They cannot cause infection, but they often need several doses or boosters to keep protection high.
Toxoid vaccines
Some bacteria cause harm mainly through toxins they release. Toxoid vaccines use an inactivated form of the toxin, teaching the immune system to neutralize it. Tetanus and diphtheria vaccines are the standard examples.
Subunit and recombinant vaccines
Instead of the whole germ, a subunit vaccine uses only selected pieces of it, such as a surface protein. A landmark was the hepatitis B vaccine. Merck's first hepatitis B vaccine, Heptavax-B, approved in the United States in 1981, was made from proteins purified from human blood plasma. On 23 July 1986 the FDA approved a version made by inserting the gene for the hepatitis B surface protein into yeast cells (Saccharomyces cerevisiae), which then produced the protein. It was the first human vaccine produced by recombinant DNA methods, and it removed the need for human blood products. The HPV vaccine Gardasil is another subunit vaccine.
Conjugate vaccines
Some bacteria are covered with a sugar coat (polysaccharide) that the immune systems of young children respond to poorly. The Haemophilus influenzae type b (Hib) story shows why conjugate vaccines were invented. A plain polysaccharide Hib vaccine was first marketed in the United States in 1985, but it gave variable, short-lived responses and children under 18 months responded poorly. In 1987 the United States licensed the first conjugate Hib vaccine, in which the sugar was chemically attached to a protein carrier (diphtheria toxoid). Linking the sugar to a protein recruits T cells, producing stronger responses and lasting memory, even in infants. In the United States, rates of invasive Hib disease fell from 40 to 100 per 100,000 children to fewer than 1 per 100,000, and countries using the vaccine routinely have seen severe Hib infections fall by more than 90 percent. Even unvaccinated young children benefited, because vaccinated children carried the bacteria less often.
Viral vector vaccines
These use a harmless, modified virus as a delivery vehicle to carry genetic instructions for an antigen into cells. Several COVID-19 vaccines used this design, including the Johnson & Johnson (Janssen) vaccine, which received U.S. emergency use authorization on 28 February 2021, and the Oxford–AstraZeneca vaccine, which was authorized in the United Kingdom in December 2020.
Nucleic acid vaccines: mRNA and DNA
mRNA vaccines deliver a strand of messenger RNA that encodes an antigen. The body's own cells read the instructions and briefly make the antigen, which then trains the immune system. The mRNA is translated in the cytoplasm, does not enter the cell nucleus or alter the cell's DNA, and is broken down within a few days. Because naked mRNA is fragile, it is usually packaged in tiny fat bubbles called lipid nanoparticles, which protect it and help it enter cells. DNA vaccines follow a similar logic but deliver DNA instead.
A short history of mRNA vaccines
mRNA vaccines seemed to appear suddenly in 2020, but the technology had been developing for about three decades. In 1989 researchers successfully delivered mRNA packaged in a liposome into cells, and in 1990 scientists injected naked mRNA into mouse muscle and saw protein produced. Those studies led to the concept of an mRNA vaccine.
Early mRNA triggered unwanted inflammation and did not produce enough protein. Work by Katalin Karikó and Drew Weissman on chemically modified nucleosides, the building blocks of RNA, helped solve that problem. They shared the 2023 Nobel Prize in Physiology or Medicine for that work. Delivery was the other challenge. The FDA first approved a lipid nanoparticle delivery system in 2018, for the RNA drug Onpattro, and that encapsulation technology became a crucial part of the COVID-19 vaccines. The first human trial of an mRNA vaccine against an infectious disease, rabies, began in 2013.
On 2 December 2020 the United Kingdom's Medicines and Healthcare products Regulatory Agency (MHRA) became the first regulator to approve an mRNA vaccine, the Pfizer–BioNTech COVID-19 vaccine. The U.S. FDA issued an emergency use authorization for that vaccine on 11 December 2020 and for the Moderna vaccine on 18 December 2020.
Storage became a practical issue. The Pfizer–BioNTech vaccine originally had to be kept between −80 and −60 °C, which required specialized freezers. Moderna's vaccine could be stored between −25 and −15 °C and was stable at 2 to 8 °C, ordinary refrigerator temperature, for up to 30 days. Storage requirements like these strongly affect how easily a vaccine can be distributed, particularly in places with limited cold-chain infrastructure.
Step one: research and discovery
According to the U.S. Centers for Disease Control and Prevention (CDC), vaccine development begins with a research and discovery phase that often involves 10 to 15 years of laboratory research. This work is usually done in private industry, sometimes in partnership with university researchers. Scientists study the germ, work out which antigens might trigger a protective response, and decide which vaccine platform to use.
Choosing the right target is not always straightforward. For influenza, the surface proteins of the virus change constantly, so the vaccine has to be updated. For some diseases, such as HIV, decades of research have not yet produced an effective licensed vaccine. Overall development is long and expensive: the Wikipedia overview of vaccines notes that development typically takes 10 to 15 years and that building a manufacturing plant can cost $50 million to $300 million.
Step two: preclinical and proof-of-concept studies
Before a vaccine can be tested in people, researchers test it in the laboratory and in animals. The CDC describes this as the proof-of-concept stage, in which researchers test the immune response in small animals such as mice and may adjust the vaccine before human testing begins.
Preclinical studies aim to answer basic questions. Does the vaccine provoke an immune response? Does that response protect animals against infection or disease? What dose might be appropriate? Are there signs of toxicity? Researchers also look for warning signs specific to a disease. For some viruses, a poorly designed vaccine can produce antibodies that worsen a later infection rather than preventing it, so this risk is examined carefully.
To move into human trials in the United States, the vaccine's sponsor must submit an Investigational New Drug (IND) application to the Food and Drug Administration (FDA). The application describes the vaccine, how it is made, the preclinical results and the plan for testing in people.
Step three: clinical trials in people
Human testing proceeds in phases, each larger than the last. The CDC describes the stages like this:
- Phase 1 involves about 20 to 100 people. The main focus is safety, side effects and whether the vaccine produces an immune response.
- Phase 2 involves hundreds of people, often 100 to 300, who share characteristics, such as age and health status, with the people the vaccine is intended for. It adds information on safety, immune response, dose and schedule.
- Phase 3 involves thousands of people, commonly 1,000 to 3,000 and sometimes many more. It confirms whether the vaccine actually prevents disease and monitors both common and less common side effects.
- Phase 4 takes place after approval. These are formal, ongoing studies of long-term safety and effectiveness in large populations.
Phase 3 trials are often randomized and controlled: some participants get the vaccine and others get a placebo or a comparison vaccine, and researchers then compare how many people in each group get sick. Because a trial only includes a limited number of people, very rare side effects may not show up until much larger numbers are vaccinated. That is why monitoring after approval is treated as part of development, not an afterthought.
Clinical trials also follow ethical rules. Participants must give informed consent, and independent boards review trial plans and safety data. Many of these protections exist because of past abuses and mistakes in medical research.
Step four: manufacturing
A vaccine that works in a trial is not useful unless it can be made reliably, at scale, with every batch matching the one that was tested. Manufacturing is often one of the hardest parts of vaccine development.
Production methods differ by vaccine type. Influenza vaccine is a clear example. Most flu vaccines are still made with the traditional egg-based method: fertilized chicken eggs are inoculated with the virus, incubated, and then the virus is harvested from the fluid. Up to three eggs may be needed for one trivalent dose. In 2012 the FDA approved the first cell-culture influenza vaccine, in which the virus is grown in cells rather than eggs, and in 2013 it approved Flublok, a recombinant vaccine made from proteins rather than whole virus. The recombinant hepatitis B vaccine is produced in yeast. mRNA vaccines are made by chemical and enzymatic processes rather than by growing a germ; scaling up their production was a major challenge, especially for the specialized ionizable lipids needed in large quantities.
In the United States, the FDA reviews the proposed manufacturing process during Phase 3 and inspects the production facility. Vaccines are made in batches called lots, and these lots are tested for consistency. Manufacturers must keep submitting lot test data even after approval, and the FDA continues to inspect facilities and test for safety, purity and potency.
Step five: regulatory approval and recommendations
In the United States, vaccines are regulated by the FDA's Center for Biologics Evaluation and Research (CBER). To seek approval, a company submits a Biologics License Application (BLA) containing clinical and preclinical data, manufacturing details, information about the facility, and the proposed prescribing information. The FDA's Vaccines and Related Biological Products Advisory Committee (VRBPAC) sometimes reviews the data in a public meeting and gives the agency advice.
FDA approval means a vaccine may be sold. Who should actually receive it is a separate question. In the United States, the Advisory Committee on Immunization Practices (ACIP), a group of medical and public health experts, considers approved vaccines and makes recommendations. ACIP looks at safety and effectiveness by age group, how severe the disease is, and the expected public health benefit. The CDC director decides whether to adopt ACIP's recommendations. Once adopted, they become official CDC guidance and may be added to the immunization schedules.
During public health emergencies, the FDA can also issue an Emergency Use Authorization (EUA), which allows a product to be used before full approval when the available evidence supports it. Several COVID-19 vaccines were first made available in the United States this way.
Other countries have their own regulators, such as the MHRA in the United Kingdom. Internationally, the WHO "prequalifies" vaccines, a review that allows them to be purchased and distributed by UN agencies such as UNICEF. This matters for many low- and middle-income countries that rely on these programs.
Step six: watching for problems after approval
Vaccine monitoring continues for as long as a vaccine is in use. In the United States, the CDC describes several systems:
- VAERS (Vaccine Adverse Event Reporting System) is an early warning system run jointly by the CDC and FDA. Anyone, including patients, parents and health care workers, can report a health problem that happened after vaccination. A VAERS report does not prove that the vaccine caused the problem; the system is designed to spot unusual patterns that deserve investigation.
- The Vaccine Safety Datalink (VSD) is a collaboration between the CDC and health care organizations that uses medical records for ongoing monitoring and active searches for possible safety problems.
- The Clinical Immunization Safety Assessment (CISA) Project is a CDC partnership with medical centers that carries out clinical research on vaccine-related risks in specific groups.
The history of the first rotavirus vaccine shows how this monitoring works in practice. RotaShield, made by Wyeth, was licensed in the United States in 1998 after trials in the United States, Finland and Venezuela found it 80 to 100 percent effective against severe rotavirus diarrhea. After licensure, studies linked it to intussusception, a serious bowel blockage. Infants were about 30 times more likely to develop the condition in the 3 to 7 days after the first dose, with an estimated excess risk of roughly one case for every 5,000 to 10,000 vaccinated infants. ACIP withdrew its recommendation, and the manufacturer pulled the vaccine from the market in 1999. Newer rotavirus vaccines followed: RotaTeq, approved by the FDA in February 2006, and Rotarix, approved in the European Union in 2006 and in the United States in April 2008.
The dengue vaccine Dengvaxia offers a different lesson. It was first sold in 2016 in 11 countries. In November 2017 its maker, Sanofi, announced that people who had never been infected with dengue could face a higher risk of severe dengue after vaccination. The company recommended using it only in people who had already had dengue. The WHO advised that the vaccine be considered only where pre-vaccination screening, or very high levels of past infection in the community, could reduce the risk to people never previously infected. In the Philippines, where more than 733,000 children had already been vaccinated regardless of prior infection, the government suspended its mass vaccination program, and the episode became a long-running political controversy. Dengvaxia was later approved in the European Union in December 2018 and in the United States in May 2019, with use limited to people with laboratory-confirmed prior infection. A different dengue vaccine, Takeda's Qdenga, was approved in Indonesia in August 2022 and in the European Union in December 2022.
Updating vaccines: influenza as a model
Some vaccines need regular updating because the germ changes. Influenza is the best-known case. The WHO's Global Influenza Surveillance and Response System collects virus samples from national centers around the world and sends them to WHO collaborating centers for analysis. Since 1999 the WHO has issued strain recommendations twice a year, one for the Northern Hemisphere flu season and one for the Southern. Manufacturers then produce vaccines matching the recommended strains. This yearly cycle shows how surveillance, laboratory science and manufacturing have to work together on a fixed timetable.
Lessons from COVID-19
The COVID-19 pandemic put vaccine development under extraordinary pressure and changed how many people think about it. The first COVID-19 vaccines were authorized less than a year after the virus was identified. That speed came from several factors.
First, the underlying technology already existed. mRNA and viral vector platforms had been studied for years, so researchers could insert the new virus's genetic information into a known system.
Second, steps that normally happen one after another were run in parallel. In the United States, Operation Warp Speed, first reported on 29 April 2020 and officially announced on 15 May 2020, was a public-private partnership to speed up the development, manufacturing and distribution of COVID-19 vaccines, treatments and tests. Its stated goal was to deliver 300 million doses of safe and effective vaccine, with initial doses by January 2021. Funding started at about $10 billion and rose to about $18 billion by October 2020. The program backed several vaccine candidates at once and paid for mass manufacturing to begin before clinical results were known, accepting that some candidates would fail. Moderna received $1.53 billion, Johnson & Johnson $1 billion and Novavax $1.6 billion, among others. Pfizer received a $2 billion advance purchase order for 100 million doses but said it did not take development funding from the program.
Third, money and government commitment reduced the financial risk that normally slows development, because companies did not have to wait for proof of success before building production capacity.
The pandemic also exposed problems. By 31 January 2021, 63.7 million doses had been delivered in the United States, and the rollout was criticized for poor coordination between federal and state governments and for slow administration. Around the world, getting vaccines to every country quickly proved far harder than inventing them, and in 2021 governments and the World Trade Organization debated whether to waive intellectual property rights on COVID-19 vaccines.
Preparing for the next outbreak
Planning for future epidemics now focuses on reducing the time from a new threat to a usable vaccine. The Coalition for Epidemic Preparedness Innovations (CEPI) was conceived in 2015 and launched in January 2017 at the World Economic Forum in Davos. Its founders included the Wellcome Trust, the Bill & Melinda Gates Foundation, the governments of Norway and India, and the World Economic Forum. CEPI funds research on vaccines against emerging infectious diseases, including those on the WHO's list of priority pathogens. In 2022 it adopted a goal, often called the "100 days mission," of being able to respond to a new pandemic threat with a vaccine within 100 days.
Reaching that goal depends on the same pipeline described in this article: early research on families of dangerous viruses, ready-made platforms, trial networks that can start quickly, manufacturing capacity that can be switched on, regulators prepared to review data rapidly, and safety monitoring that continues after rollout.
The impact of vaccines
The long-term results of this work are large. A study led by the WHO and published in The Lancet in 2024 estimated that global immunization efforts had saved at least 154 million lives over the previous 50 years, since the World Health Assembly founded the Expanded Programme on Immunization in 1974. Of those lives, 101 million were infants. Measles vaccination alone accounted for about 60 percent of the lives saved, and vaccination against the 14 diseases studied was estimated to have cut infant deaths by about 40 percent worldwide.
Individual diseases tell the same story. In 1958 the United States recorded 763,094 measles cases and 552 deaths; after vaccines were introduced, cases fell to fewer than 150 per year. Smallpox is gone, and wild polio transmission has been pushed back to a small number of countries.
Frequently asked questions
How long does it normally take to develop a vaccine? Traditionally, many years. The CDC notes that research and discovery alone often take 10 to 15 years, and the full process includes animal studies, three phases of human trials, manufacturing scale-up and regulatory review. COVID-19 vaccines were faster because platforms already existed, funding was large, and steps such as manufacturing were run in parallel with trials rather than after them.
Were COVID-19 vaccine trials skipped to save time? No phases were skipped. The vaccines went through Phase 1, 2 and 3 trials. Time was saved mainly by overlapping steps and by starting manufacturing before results were known, with governments taking on the financial risk.
Can an mRNA vaccine change your DNA? No. The mRNA works in the cytoplasm of the cell, does not enter the nucleus where DNA is kept, and is broken down within a few days.
What is the difference between FDA approval and a CDC recommendation? FDA approval means a vaccine has been judged safe and effective enough to be licensed. A recommendation from ACIP, adopted by the CDC director, decides which groups of people should receive it and when, and puts it on the immunization schedule.
Why are some vaccines given every year? Some germs, especially influenza viruses, change quickly. The WHO reviews circulating flu strains twice a year and recommends which ones the vaccine should target, so the vaccine is updated to keep up.
How are rare side effects found if trials only include thousands of people? Through monitoring after approval. Systems such as VAERS and the Vaccine Safety Datalink in the United States look for unusual patterns across millions of doses. The withdrawal of the RotaShield rotavirus vaccine in 1999 is an example of this system detecting a rare risk.
Sources and further reading
- Wikipedia, "Vaccine": https://en.wikipedia.org/wiki/Vaccine
- Wikipedia, "Smallpox vaccine": https://en.wikipedia.org/wiki/Smallpox_vaccine
- Wikipedia, "Louis Pasteur": https://en.wikipedia.org/wiki/Louis_Pasteur
- Wikipedia, "Polio vaccine": https://en.wikipedia.org/wiki/Polio_vaccine
- Wikipedia, "Hepatitis B vaccine": https://en.wikipedia.org/wiki/Hepatitis_B_vaccine
- Wikipedia, "Hib vaccine": https://en.wikipedia.org/wiki/Hib_vaccine
- Wikipedia, "mRNA vaccine": https://en.wikipedia.org/wiki/MRNA_vaccine
- Wikipedia, "Influenza vaccine": https://en.wikipedia.org/wiki/Influenza_vaccine
- Wikipedia, "Rotavirus vaccine": https://en.wikipedia.org/wiki/Rotavirus_vaccine
- Wikipedia, "Dengue vaccine": https://en.wikipedia.org/wiki/Dengue_vaccine
- Wikipedia, "Operation Warp Speed": https://en.wikipedia.org/wiki/Operation_Warp_Speed
- Wikipedia, "Coalition for Epidemic Preparedness Innovations": https://en.wikipedia.org/wiki/Coalition_for_Epidemic_Preparedness_Innovations
- U.S. Centers for Disease Control and Prevention, "How Vaccines are Developed and Approved for Use": https://www.cdc.gov/vaccines/basics/test-approve.html
- World Health Organization, "Global immunization efforts have saved at least 154 million lives over the past 50 years" (24 April 2024): https://www.who.int/news/item/24-04-2024-global-immunization-efforts-have-saved-at-least-154-million-lives-over-the-past-50-years