So You’ve Been Vaccinated. When Are You Safe?

Photo-Illustration: by Intelligencer; Photo Getty Images

If you’ve recently received a COVID-19 vaccination, congratulations — but you’re not out of the woods. Right after your first dose, you’re as vulnerable to infection as you were before, since it takes time for your immune system to learn how to find and kill the coronavirus. When that happens, recipients of the Pfizer, Moderna, and Johnson & Johnson vaccines will all enjoy nearly 100 percent protection against hospitalization and death. So when can you start to feel safe? Sooner than you might think.

Pfizer and Moderna, the only vaccines to have published their Phase 3 trial results in a peer-reviewed journal, are both based on mRNA technology, and as might be expected showed similar trajectories. During the first two weeks after the first dose, recipients were almost as likely as control subjects given a placebo to develop COVID-19. But after that, the vaccinated infection rate dropped quickly. Pfizer’s data is more detailed, and it’s striking. Before day 11, vaccination offers very little protection. After day 11, the protection is substantial. Given the overall similarity in efficacy, presumably Moderna is the same way. (Johnson & Johnson’s vaccine uses a different technology, and detailed results of their Phase 3 study haven’t been published yet, so it’s harder to say what’s going on there.)

Rachel Roper, a professor of microbiology at East Carolina University, says this timing isn’t surprising. The body makes millions of white blood cells that are randomly shaped to match all possible unknown things. “Most just lie dormant and die, but if one binds a virus or other pathogen, it will become activated, and then proliferate, and then differentiate to be an effector cell, making antibodies or becoming a killer cell.  This is why it takes a week or two to get over an illness.  It takes time for these very few specific cells to activate and proliferate to large enough numbers to control infection.”

Looking at the Pfizer graph, the protection level is nearly binary: none, and then all. Day 11 after the first shot seems to be the magic point beyond which you can consider yourself vaccinated.

Graphic: The New England Journal of Medicine

But maybe not? According to a recent article by the CDC, vaccine recipients should only consider themselves “partially protected” until two weeks after their second dose, at which point they can consider themselves “fully protected.” Only at this point, per the CDC, can they engage in a number of activities like traveling by air (with a mask) or visiting unmasked people.

Their stated justification is that in a real-world study of 3,950 frontline workers, Moderna and Pfizer were found to provide 80 percent protection from infection during the “partially protected” phase, whereas after that they provided 90 percent protection. (The study counted asymptomatic as well as symptomatic cases.)

According to George Rutherford, a professor of epidemiology at the University of California at San Francisco, it does make sense that as time goes by, a vaccine should slowly increase the body’s level of protection. “You continue to get higher and higher levels of antibody,” he says.

But he agrees that there is nothing visible in the data that suggests some kind of demarcation at two weeks after the second dose. “I don’t see another inflection point,” says Rutherford.

So if you decide to consider yourself “fully protected” two weeks after your first dose, or decided to follow the CDC’s advice and wait another month, there’s no statistical evidence that it will make much of a difference.

It cannot be emphasized enough, however, that a vaccination does not render anyone immune to COVID. The Phase 3 trials were conducted at a time when everyone was being advised to mask up and socially distance. So everyone should continue to take reasonable precautions until herd immunity kicks in.

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