You’re Not Fully Vaccinated the Day of Your Last Dose

a grasshopper waits on a vaccine syringe
Wikimedia / The Atlantic

For much of 2020, the world pinned its collective post-pandemic plans on a single, glimmering end point: the arrival of an effective COVID-19 vaccine. The resounding refrain of “when I’m vaccinated” has long conjured images of people shedding their masks, hugging their friends, and returning to a semblance of normalcy. And now some vaccinated people are doing exactly that. In recent weeks, I’ve heard dozens of stories from friends, family members, and co-workers about vaccinees who are immediately dropping their guards after their shots, in some cases discarding their masks and congregating with others. Deepta Bhattacharya, an immunologist at the University of Arizona, told me that one of his colleagues—another biologist—went out to a celebratory dinner right after getting a dose at a 24-hour clinic.

But immunity to the coronavirus doesn’t just magically manifest the day someone gets a shot. The CDC does not grant membership to the “fully vaccinated” club until at least two weeks after the final dose in a vaccine regimen—a time that roughly corresponds to when most people are thought to acquire enough immunity to defend against a symptomatic case of COVID-19. Only then, the agency announced last week, can vaccinees start to carefully change their behavior, mingling maskless in small groups indoors, visiting the unvaccinated on a limited basis, and skipping postexposure quarantines.

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The jab itself “is momentous,” said Bhattacharya, who shared photos of his own injections on Twitter in hopes of swaying hesitant peers. But each injection is merely a precursor of what’s to come. “Ultimately, the real momentous ‘occasion’ is what happens gradually after the [final] shot,” he said.

This is true of all vaccines: Their protective effects take several days or weeks to kick in. It’s the reason we get our flu shots in the fall, well before the height of respiratory-virus season, and it’s why health officials often recommend that vaccines required for travel, such as those that ward off yellow fever, be administered about a month or more in advance. Vaccination, and the defenses it affords, is less a singular event than a series of steps on a shifting landscape.

From the standpoint of protection, not a whole lot happens on day one of a vaccination regimen—which makes concern about infections detected around the time of vaccination unwarranted. In late December and January, social-media platforms were swarmed by a flurry of nervous headlines and sound bites documenting positive test results in recently injected health-care workers and politicians. But cases like these are entirely expected. The shot simply delivers a package of study materials to the body; immune cells must then internalize the information about the infectious invader, a complex process that unfolds over days or weeks.

Shortly after the vaccine is administered, these cells embark on a crash course in the coronavirus. Fast-acting immune cells inspect the shot’s contents, then ferry the intel back to their specialized colleagues: B cells, the immune cells that make antibodies, and T cells, which can annihilate virus-infected cells, learn to zero in on the pathogen with laser-sharp precision.

The body also works hard to ensure that only the best B cell and T cell fighters are recruited to the cause. Some of these cells will even compete with one another, eliminating the weaker or less discerning fighters. “They need to be able to recognize when they should respond, and when they should leave well enough alone,” Bhattacharya said. “That takes some time.”

In this light, a SARS-CoV-2 infection that occurs before the body has had time to respond to a vaccine is about as surprising as students failing an end-term exam because they haven’t finished the reading assignments. (Some asymptomatic infections are also expected with the currently cleared vaccines, which are expected to be better at protecting against symptomatic disease.)

Vaccine-induced protection also endures and evolves as the pathogen-memorizing pupils of the immune system crunch through their lessons. There’s nothing particularly special about day 14; antibody levels ramp up gradually after a shot or series of shots is delivered, Padmini Pillai, an immunologist at MIT, told me. But data collected during the vaccine makers’ clinical trials indicate that after two weeks, the body reaches a “threshold of protection,” Pillai said. (It’s worth mentioning here that the Pfizer-BioNTech and Moderna vaccines require boosters three or four weeks after the initial injection; the Johnson & Johnson jab, which contains very different ingredients, seems to be memorable enough as a single shot.)

Although plenty of people have documented their shots on camera, vaccinees aren’t taking many selfies at the two-week mark: The end of a waiting period is a pretty dull milestone, especially compared with the photogenic pizzaz of the injection itself, which comes complete with a needle-tipped syringe filled with lifesaving liquid. The day on which people are cleared to alter their behavior is, by contrast, devoid of “salient, concrete cues,” says Gretchen Chapman, a psychologist at Carnegie Mellon University who studies human behavior and vaccines. “Two weeks later, nothing happens; there’s no event,” she told me. “It’s not like your arm turns purple to tell you, It’s time.

Considering the nation’s sputtering rollout and the many logistical problems that have stymied delivery, getting any shot at all is worthy of celebration. But there’s a gentle deception tucked into fixating on the moment a vaccine enters an arm: It runs the risk of conflating the time of injection with the time of protection.

Clearer messaging about the vaccine timeline might help, says Alison Buttenheim, a behavioral scientist at the University of Pennsylvania who studies vaccines. Receiving the second shot in a two-dose vaccine regimen, for example, counts as series completion, Buttenheim told me. That’s not the same thing as what the CDC, in its new guidelines, means by “fully vaccinated.”

And yet, many news outlets and public-health authorities, including the CDC’s own vaccination tracker, are putting people in the “fully vaccinated” count as soon as they receive their second shot of the Moderna or Pfizer vaccine, or their one-and-done dose of Johnson & Johnson. (The agency acknowledges in a footnote at the bottom of its tracker that it is using fully vaccinated in two different ways.)

To distinguish injection from protection, vaccination cards could include the dates of not just the shots themselves, but the end of the recommended waiting period. Both Chapman and Buttenheim pointed out that some health departments and vaccine makers once distributed refrigerator magnets to the young recipients of HPV shots, which are delivered in multidose regimens over several months. To keep parents and their kids from forgetting their next appointment, the magnets lit up when it was time to return for a booster. If something similar were handed out at COVID-19 vaccination sites to denote when people reach the protection threshold, “wouldn’t that be nifty?” Chapman said.

Perhaps people should take a second or third vaccine selfie, commemorating their immune status after their cells are better prepared to fight off the virus. These photos wouldn’t be as flashy. But they might help paint a more realistic portrait of what vaccination actually is—not a discrete moment, but a gradual unfurling on scales large and small. Immunity builds iteratively, not instantaneously, in people; it accumulates slowly, over time, in populations. The vaccines are here. After a difficult year of waiting, we all just have to wait a little longer.