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

Reports of Covid-19 cases that appeared shortly after a single shot of a two-dose vaccine shouldn’t cause concern.

By Katherine J. Wu

Pfizer's COVID-19 Vaccine Effects

A flurry of headlines this week flooded social , 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.

Keep reading on nytimes.com…