Vaccines Take a While to Kick In. Experts Say That Means the Body Is Doing Its Job.

A flurry of headlines this week flooded social media, documenting a seemingly concerning case of Covid-19 in a San Diego nurse who fell ill about a week after receiving his first injection of Pfizer’s coronavirus vaccine.

But experts said the sickness is nothing unexpected: The protective effects of vaccines are known to take at least a couple of weeks to kick in. And getting sick before completing a two-dose vaccine regimen, they said, should not undermine the potency of Pfizer’s product, which blazed through late-stage clinical trials with flying colors.

Reporting that a half-vaccinated person has Covid-19 is “really the equivalent of saying someone went outside in the middle of a rainstorm without an umbrella and got wet,” said Dr. Taison Bell, a critical care physician at the University of Virginia. Dr. Bell received his first dose of Pfizer’s vaccine on Dec. 15, and will be getting his second shot soon.

The California nurse, identified as Matthew W., 45, in an ABC10 News report, received his first dose of Pfizer’s vaccine on Dec. 18. Six days later, according to news reports, he began to feel minor symptoms, including chills, muscle aches and fatigue. He tested positive for the virus the day after Christmas.

Dr. Megan Ranney, an emergency physician at Brown University, said this should not prompt concern. “So what????” she tweeted on Wednesday in response to a Reuters article on the nurse’s illness. “It’s a 2-shot vaccination.” Dr. Ranney received her first dose of Pfizer’s vaccine on Dec. 18.

Framing the nurse’s illness as news, Dr. Ranney said in an interview, implies that it was a departure from the expected — and that there should have been protection about a week after the first vaccine dose. That’s not the case at all.

Vaccines take at least a few days to exert their protective effects. Pfizer’s recipe is designed around a molecule called messenger RNA, or mRNA, which, once injected, enters human cells and instructs them to manufacture a coronavirus protein called spike. None of these components are infectious or capable of causing Covid-19. But they act as coronavirus mimics, teaching the body to recognize the true virus and vanquish it, should it ever come around.

The production of spike is thought to occur within hours of the first shot. But the body needs at least several days to memorize the material before it can unspool its full arsenal of defensive forces against the virus. Immune cells take this time to study up on the protein, then mature, multiply and sharpen their spike-spotting reflexes.

Data from Pfizer’s clinical trials suggests the vaccine might start safeguarding its recipients from disease around one or two weeks after the first injection. A second jab of mRNA, delivered three weeks after the first, helps immune cells commit the virus’s most prominent features to memory, clinching the protective process.

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Answers to Your Vaccine Questions

With distribution of a coronavirus vaccine beginning in the U.S., here are answers to some questions you may be wondering about:

    • If I live in the U.S., when can I get the vaccine? While the exact order of vaccine recipients may vary by state, most will likely put medical workers and residents of long-term care facilities first. If you want to understand how this decision is getting made, this article will help.
    • When can I return to normal life after being vaccinated? Life will return to normal only when society as a whole gains enough protection against the coronavirus. Once countries authorize a vaccine, they’ll only be able to vaccinate a few percent of their citizens at most in the first couple months. The unvaccinated majority will still remain vulnerable to getting infected. A growing number of coronavirus vaccines are showing robust protection against becoming sick. But it’s also possible for people to spread the virus without even knowing they’re infected because they experience only mild symptoms or none at all. Scientists don’t yet know if the vaccines also block the transmission of the coronavirus. So for the time being, even vaccinated people will need to wear masks, avoid indoor crowds, and so on. Once enough people get vaccinated, it will become very difficult for the coronavirus to find vulnerable people to infect. Depending on how quickly we as a society achieve that goal, life might start approaching something like normal by the fall 2021.
    • If I’ve been vaccinated, do I still need to wear a mask? Yes, but not forever. Here’s why. The coronavirus vaccines are injected deep into the muscles and stimulate the immune system to produce antibodies. This appears to be enough protection to keep the vaccinated person from getting ill. But what’s not clear is whether it’s possible for the virus to bloom in the nose — and be sneezed or breathed out to infect others — even as antibodies elsewhere in the body have mobilized to prevent the vaccinated person from getting sick. The vaccine clinical trials were designed to determine whether vaccinated people are protected from illness — not to find out whether they could still spread the coronavirus. Based on studies of flu vaccine and even patients infected with Covid-19, researchers have reason to be hopeful that vaccinated people won’t spread the virus, but more research is needed. In the meantime, everyone — even vaccinated people — will need to think of themselves as possible silent spreaders and keep wearing a mask. Read more here.
    • Will it hurt? What are the side effects? The Pfizer and BioNTech vaccine is delivered as a shot in the arm, like other typical vaccines. The injection into your arm won’t feel different than any other vaccine, but the rate of short-lived side effects does appear higher than a flu shot. Tens of thousands of people have already received the vaccines, and none of them have reported any serious health problems. The side effects, which can resemble the symptoms of Covid-19, last about a day and appear more likely after the second dose. Early reports from vaccine trials suggest some people might need to take a day off from work because they feel lousy after receiving the second dose. In the Pfizer study, about half developed fatigue. Other side effects occurred in at least 25 to 33 percent of patients, sometimes more, including headaches, chills and muscle pain. While these experiences aren’t pleasant, they are a good sign that your own immune system is mounting a potent response to the vaccine that will provide long-lasting immunity.
    • Will mRNA vaccines change my genes? No. The vaccines from Moderna and Pfizer use a genetic molecule to prime the immune system. That molecule, known as mRNA, is eventually destroyed by the body. The mRNA is packaged in an oily bubble that can fuse to a cell, allowing the molecule to slip in. The cell uses the mRNA to make proteins from the coronavirus, which can stimulate the immune system. At any moment, each of our cells may contain hundreds of thousands of mRNA molecules, which they produce in order to make proteins of their own. Once those proteins are made, our cells then shred the mRNA with special enzymes. The mRNA molecules our cells make can only survive a matter of minutes. The mRNA in vaccines is engineered to withstand the cell’s enzymes a bit longer, so that the cells can make extra virus proteins and prompt a stronger immune response. But the mRNA can only last for a few days at most before they are destroyed.

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