Why Covid-19 Vaccines Take a While to Kick In

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.

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, Dr. Ranney said. It’s 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, if they ever have symptoms at all.

A similar situation appears to have recently unfolded with Mike Harmon, the Kentucky state auditor, who this week tested positive for the virus the day after receiving his first dose of an unspecified coronavirus vaccine.

“It appears that I may have been unknowingly exposed to the virus and infected either shortly before or after receiving the first dose of the vaccine on Monday,” Mr. Harmon said in a statement. Mr. Harmon reaffirmed his “full faith in the vaccine itself, and the need for as many people to receive it as quickly as possible.”

Jerica Pitts, a spokeswoman for Pfizer, noted that the vaccine’s protective effects are “substantially boosted after the second dose, supporting the need for a two-dose vaccination series.”

“Individuals may have contracted disease prior to or right after vaccination,” she said.

Pfizer’s vaccine, when administered in its full two-dose regimen, was found to be 95 percent effective at preventing symptomatic cases of Covid-19 — a figure that was hailed as very welcome news amid soaring coronavirus caseloads. Still, that leaves a small percentage of people who won’t be protected after vaccination, Dr. Ranney said. “There’s no vaccine that’s 100 percent effective.”

It’s also unclear how well Pfizer’s vaccine can guard against asymptomatic infections, or if it will substantially curb the coronavirus’s ability to spread from person to person. That means measures like masking and distancing remain essential even after full vaccination.

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