How Long After the COVID-19 Vaccine Is It Effective?

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.)

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