Pfizer / BioNTech and Moderna’s COVID-19 vaccines have a lot in common: they are both highly effective in clinical trials, they are both gene-based vaccines, and each also requires two doses. That last point is pretty standard for vaccines. “If you look at all the FDA approved vaccines, the vast majority will require multiple doses,” said Otto Yang, an infectious disease specialist at UCLA Health. That the reason why some of the most promising coronavirus vaccine candidates are no exception.
Vaccines work by exposing the body to a small portion of the virus so the immune system can learn to recognize it. More than one dose means more opportunity for the immune system to figure out exactly how to fight future infections. The immune system needs extra exposure time to learn how to effectively fight the virus because, as Yang explains, vaccines don’t replicate in the body the way viruses do.
“When you’re first getting exposed to something,” Yang said, “the immune system is actually starting from scratch.” Pathogens, such as the coronavirus, have specific areas called antigens, which trigger our cells to produce antibodies to fight infection. COVID-19 Vaccine triggers the production of specific antibodies that can fight something like a virus.
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Multiple doses of the vaccine allow the body to produce more antibodies. They also provide the body with a strong supply of memory cells, which remain in the body long after exposure. These cells readily respond to the specific antigen if it appears again. With multiple doses, the body is exposed to more antigens, so more memory cells are created, which leads to a faster and more effective antibody response in the future.
Memory cells don’t last forever and will die over time. This is why people need booster shots to maintain an immune response against infections like tetanus and diphtheria. Booster, as the name suggests, is a predefined response booster. “A booster is often just one shot because that’s enough to basically wake up the response as opposed to making a new response.” But some of the starting doses differ from booster injections. The reason for those multiple doses, Yang says, “is because generating the initial response is harder than reviving the response that’s already there.”
Two doses are the best way to make an effective count of antibodies and memory cells, but the requirement poses a logistical problem. That means twice as much material — needles, bottles, the vaccine itself — needs to be produced, stored, and distributed. It’s also harder to get people to get the COVID-19 vaccine twice. It can be very difficult for people to take time away from work, organize child care, or travel some distance to the nearest vaccination site.
We are still in the early stages of developing a vaccine for this particular virus. Although the current Pfizer / BioNTech and Moderna vaccines both require two doses, there are still dozens of other vaccines that have yet to reach the next phase of trials. Johnson & Johnson is testing the efficacy of their single and two-dose vaccines to evaluate and compare the long-term efficacy of the two. There may be simpler solutions at hand, but for now, it looks like the first available vaccine will likely come in two shots.