Multiple doses of a vaccine give the body a chance to produce more antibodies. They also give the body a robust supply of memory cells, which remain in the body long after exposure. These cells are ready to respond to those specific antigens if they appear again. With multiple doses, the body is exposed to more antigens, so more memory cells are created, leading to a faster and more effective antibody response in the future.
Memory cells don’t last forever and will die off over time. This is why people need booster shots to maintain an immune response to infections like tetanus and diphtheria. A booster is, as the name implies, a boost to a response that’s already been established. “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 multiple initial doses are different from a booster shot. 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 is the best way to create an effective number of antibodies and memory cells, but the requirement poses logistical problems. It means twice as many materials — needles, vials, the vaccine itself — need to be produced, stored, and distributed. It’s also harder to get people to go get a vaccine twice. It could be especially difficult for people to take time from work, arrange for child care, or travel some distance to the nearest vaccination site.
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We’re still in early stages of vaccine development for this particular virus. While Pfizer / BioNTech’s and Moderna’s vaccines both require currently two doses, there are still dozens of other vaccines that haven’t yet reached later phase trials. Johnson & Johnson is testing the efficacy of both a single dose and two doses of their vaccine to evaluate and compare the long-term efficacy of both. There may be a simpler solution on the horizon, but for now, it looks like the first vaccines available will likely come in two shots.