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.
The California nurse, identified as Matthew W., 45, in an ABC10 News report, received his first dose of Pfizer’s vaccine December 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.
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.
The timeline of the California nurse’s illness falls well within the window of post-vaccination vulnerability. It is also very likely he caught the virus right around the time he got the shot, perhaps even before. People can start experiencing the symptoms of Covid-19 between two and 14 days after encountering the coronavirus.
Pfizer’s vaccine, when administered in its full two-dose regimen, was found to be 95% effective at preventing symptomatic cases of Covid-19.