Why It Takes 2 Weeks For The Covid Vaccines To Kick In

The FDA-authorized Covid vaccines are, in some ways, like Spider-Man. 

"When Peter Parker gets bitten by a radioactive spider, he's not able to climb walls right away," said Dr. Vince Silenzio, an M.D. and professor in the Rutgers School of Public Health. Similar to the bite that endows Parker with certain spider-like superpowers, the Covid vaccines — which afford people a high level of protection against this still relatively new human disease — take a period of time to kick in. "You have to wait for it to really get there," emphasized Dr. Silenzio.

How long? Whether the second shot of the Pfizer-BioNTech and Moderna vaccines or the single shot Johnson & Johnson vaccine (which the FDA is currently looking into out of "an abundance of caution" regarding an extremely rare potential safety concern involving blood clots), infectious disease experts say to wait two weeks until you can call yourself fully vaccinated. As of April 2, 2021, being fully vaccinated means you can gather indoors without masks with small groups of vaccinated people and travel domestically without having to quarantine, says the CDC. (But still wear a mask if you're visiting anyone with a high risk of severe Covid illness, like someone with a lung disease).

Waiting these two weeks is crucial. For all the vaccines, our immune systems aren't as prepared to stop an infection or avoid severe disease (the type that sends people to the hospital or kills) until those two weeks elapse. Symptoms are less likely in those who are fully vaccinated, too: In real-world data (not clinical trial numbers) recently published for the Pfizer-BioNTech and Moderna vaccines, fully vaccinated people were 90 percent less likely to get infected and show any symptoms than unvaccinated people. The number dropped to 80 percent for those with one dose (but at least 14 days after the first dose).

So give it two weeks. "There’s no question that [two-weeks] is real and the recommendation is a solid one," said Dr. Silenzio.

Why it takes two weeks

If we're patient, the FDA-authorized vaccines are all excellent at preventing disease (though Johnson and Johnson may have some safety concerns. That's still TBD. Stay tuned).

"The efficacy of these vaccines is outstanding, but that’s not realized until two weeks after the vaccination," said Dr. Thomas Russo, the chief of infectious disease at the University of Buffalo's Jacobs School of Medicine and Biomedical Sciences.

Why two weeks? "When you get vaccinated, your immune system gets a to-do list," explained Mark Cameron, an immunologist at Case Western Reserve University who previously helped contain the outbreak of another deadly coronavirus, SARS, in 2003. What does the immune system need to do with the vaccine?

(All three FDA-authorized vaccines don't contain the actual virus, but genetic code showing our cells how to create just a small, specific part of the virus. Specifically, our cells make the virus' spike protein, which is designed to bind to, and ultimately infect, our cells.)

1. Our immune system will gradually recognize the spike protein (produced by the vaccine) as an intruder.

2. In response to recognizing this foreign spike protein, the bodies' immune cells will cooperate to start producing protective proteins, called antibodies, to protect you against the virus. If you're infected, these antibodies bind to the spike proteins of the virus, making it difficult or impossible for the virus to bind and gain access to our cells. (When inside, the virus hijacks our cellular machinery to mass multiply. It's an effective parasite).

"It's around the two-week mark that the immune system is producing antibodies at levels that block infections," said Cameron.

So far, there's good news about how long these antibodies last. By six months after the second Moderna shot, all age groups in a clinical trial showed high antibody levels. And six months after the second Pfizer dose in a large-scale trial, the vaccine reduced severe disease by over 95 percent, meaning antibodies are likely working quite well.

(The vaccine also triggers other parts of the immune system to develop longer-term protection against the coronavirus. They're called memory T and B cells, and they have the ability to store the "memory" of the spike protein in our immune systems, in case the virus enters the body again. "They can react to later infections and start up the antibody construction again," explained Cameron, who noted that researchers are still investigating how effective this component of Covid immunity is, and how long it might last.) 

In the case of the two-shot vaccines, the first dose is the "priming" shot that "wakes up your immune system," explained Dr. Russo. Then a few weeks or a month later (depending on the vaccine), the second shot really ramps up the immune response, which triggers a surge in antibody production. "The second shot increases antibodies ten-fold," explained Dr. Russo. 

While all the FDA-authorized vaccines will provide high levels of protection against a Covid infection, they won't make you completely invincible, emphasized Rutgers' Dr. Silenzio. Spider-Man, while powerful, wasn't invincible, either. The vaccines, however, don't just make symptoms unlikely. They vastly reduce the likelihood of severe disease, hospitalization, and death. In clinical trials involving tens of thousands of participants, all three vaccines resulted in zero hospitalizations and deaths, though such serious "breakthrough infections" (meaning an infection after full vaccination) are likely to sometimes occur in the real world, where millions are getting vaccinated daily.

"Vaccinate and wait."

Waiting for the vaccines to kick in is crucial because infection numbers remain high in the U.S., with extreme outbreaks in certain places like Michigan. And when the virus makes people seriously ill for weeks at a time, it continues replicating by the millions and, inevitability, mutating. That's how potentially more transmissible, partially-vaccine resistance coronavirus variants form. "We're giving this virus plenty of opportunity," warned Cameron. 

"Vaccinate and wait," he emphasized.