How do vaccines work? The scientific process behind how vaccines are made

Vaccines work by exposing your immune system to parts of a virus or bacteria so it can learn how to protect your body against that germ.
Vaccines are made by growing or generating parts of a germ, which are often weakened or inactivated to safely and effectively expose your body to that virus or bacteria.
Here’s what you need to know about how long it takes to develop a vaccine and the six-step process behind how they’re made.
This article was medically reviewed by Alex Berezow, PhD, a microbiologist at the American Council on Science and Health.
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Vaccines are vital to fighting disease, not only for each individual, but also for entire communities. For reference, the World Health Organization suggests that vaccines prevented at least 10 million deaths worldwide between 2010 and 2015.

However, the process of making a vaccine is difficult, complicated, and takes time. Here’s what you need to know about how different types of vaccines work and how they’re made.

How vaccines work

Vaccines use weakened, dead, or other parts of a virus to provoke the immune system into responding so it can learn how to protect your body against that virus, says Abe Malkin, MD, founder and medical director of Concierge MD in Los Angeles.

“Vaccines are made by … creating a weakened or inactivated version of a virus that isn’t strong enough to make a patient sick, but can still trigger the body to mount an immune response which creates antibodies against the virus,” Malkin says.

But not all viruses are alike. According to Malkin, the approach to creating a vaccine depends on the characteristics of the disease, how it affects the body, how the immune system reacts, and the population most affected by it.

Generally, vaccines can work in four different ways:

Live-attenuated vaccines. These use a weakened version of the virus to build immunity, and attenuation was the practice used for the first vaccinations against smallpox. This method is also used for vaccination against measles, mumps, rubella, rotavirus, chickenpox, and yellow fever.
Inactivated vaccines. These vaccines use chemicals to kill the virus, but aren't as effective as live vaccines, which is why vaccine boosters are needed - for diseases like rabies, you'll require three separate injections over a period of time. This type of vaccine is also used against hepatitis A, influenza, and polio.
Subunit, recombinant, polysaccharide, or conjugate vaccines. These vaccines specifically target parts of the germ rather than the whole bacteria or virus. These vaccines are used against hib disease, hepatitis B, HPV, whooping cough, pneumococcal disease, meningococcal disease, shingles.
Toxoid vaccines: These vaccines target a germ's toxin rather than the bacteria or virus itself, and treat diphtheria and tetanus.

Each of the vaccine types has different strengths and weaknesses, says Brendan Wren, a professor of microbial pathogenesis at the London School of Hygiene & Tropical Medicine, and choosing the right one is vital.

"Live attenuated strains are required to deliver a strong and suitable immune response, but live attenuated strains could have safety issues," Wren says. "The subunits vaccines, which are based on the cell surface of the virus or bacteria are generally safer, but may not deliver a strong enough immune response and booster vaccines may be required."

How vaccines are made

Vaccines are made by growing or generating parts of a virus, which can then be used to stimulate a response from the immune system, so it can learn how to fight off that virus.

According to a March 2020 article in The Conversation by members of the Oxford Vaccine Group, developing a vaccine is usually a six-stage process:

A basic understanding of the virus. The virus will be studied in a lab to identify its unique properties and characteristics, and how it affects human or animal cells.
Finding the vaccine candidate. After studying the virus, scientists must decide which approach to take. This could include any of the four processes described above, such as isolating the live virus to weaken or inactivate it, or using the virus's genetic sequence to then create a vaccine.
Test the vaccine in pre-clinical trials. The vaccine will be tested on animals first to determine how humans might respond to it, and researchers may adapt the vaccine here to make it more effective.
Test the vaccine in clinical trials. These tests are performed on humans, in three phases: first, on a few dozen volunteers; second, on several hundred 'target population' members who are at risk from the disease; and third, on several thousand people to determine how safe and effective it is.
Get regulatory approval. Bodies like the FDA need to approve the vaccine and examine the trial results. Policies need to be drafted about the usage of it, such as who is a priority for the vaccine.
Put the vaccine into production. Finally, the vaccine can be produced, first on a small scale and then in larger quantities, before distribution occurs.

How long it takes to develop a vaccine

The Oxford Vaccine Group, one of more than 40 research teams developing a vaccine for COVID-19, says producing an effective and safe vaccine can take between five to ten years. However, many are looking for a faster solution in the case of COVID-19.

"Given the current severity of the crisis, there are efforts to fast-track a vaccine for COVID-19 in as little as 12 to 18 months," says Malkin, although experts have warned about the dangers of pushing a vaccine through safety regulations too quickly.

During active outbreaks, research can often be expedited because of increased funding and resources. Fortunately, researchers were already working on vaccines for other variations of the coronavirus - and the genetic sequence of this novel coronavirus was shared by Chinese scientists in January - which could help speed up the process for the COVID-19 vaccine.

But Wren warns that even with the headstart on stage one, vaccine production is a long and intensive process. "To mimic an infection, by its very nature, will take months," he says.

Wren emphasizes that gaining legal and ethical permission to do even the phase one human trials can take two to three months, with the whole clinical trial process taking at least nine months.

While there are plenty of researchers dedicating their time and effort to the development of the vaccine, Wren says it will still take some time: "It seems promising, but there is no guarantee that any of the vaccines will evoke a strong enough immune response."