You got your first dose of the Covid vaccine. Now what?

Last Saturday, the U.S. hit a new high for the number of daily vaccinations: 4 million. The record day came as vaccinations have steadily risen over recent weeks, bringing the daily average to more than 3 million.

That means more and more people each day in the U.S. are receiving one dose of a Covid-19 vaccine. For the majority, who will receive either the Pfizer-BioNTech or Moderna vaccine, it’s just the first dose; in three to four weeks, they’ll be returning for a second.

But is a person protected after just the first shot?

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Only if the vaccine was the Johnson & Johnson one-dose shot. For the other two vaccines currently available in the U.S., experts say it’s essential that people who receive a two-dose vaccine come back for the second in order to be fully protected.

People who are partially vaccinated shouldn’t change their behavior and instead should sit tight until they have completed their vaccination series, said Dr. Paul Offit, director of the Vaccine Education Center at Children’s Hospital of Philadelphia and a member of the Food and Drug Administration’s vaccine advisory committee.

“It just frightens me that we have, in any sense, put out there the notion that [Pfizer and Moderna] are anything other than a two-dose vaccine,” Offit said.

Last week, the Centers for Disease Control and Prevention reported that the first dose of either the Pfizer or Moderna vaccine was 80 percent effective at preventing infection. That protection was measured two weeks after the first dose, but before the second.

During a White House Covid-19 response team briefing Monday, Dr. Anthony Fauci, the nation’s leading infectious disease expert, qualified those findings, saying, “It is somewhat of a tenuous 80 percent.”

“The question,” Fauci said, “is how long does [that protection] last?”

“The best reason to wait for that second dose is you want to get durable immunity,” Offit said.

Research from Pfizer, released last week, showed that protection lasted at least six months in people who received both doses of the vaccine.

“Getting the second dose is going to be really important for lasting protection,” said Dr. Peter Hotez, co-director of the Center for Vaccine Development at Texas Children’s Hospital and dean of the National School of Tropical Medicine at Baylor College of Medicine.

What’s more, two doses of the Pfizer vaccine — not one — appear to be protective against certain variants of the virus, including the variant that was first detected in South Africa, the drugmaker said last week.

It’s after the second dose, Offit said, that protection against variants really kicks in.

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Both Offit and Hotez said the second dose of an mRNA vaccine, like Pfizer’s and Moderna’s, elicits a much more robust immune response than a single dose.

The second dose signals to the immune system that the coronavirus is something worth mounting a long-term response to, said Michal Tal, an immunologist at Stanford University.

“For the immune system, it really does matter if you continue to see something,” she said, referring to the second dose. “If you see it again, that’s a signal that this is something worth dedicating a lot of energy to and making a strong memory response against.”

The single-dose Johnson & Johnson vaccine, on the other hand, leads to a similarly robust immune response after just one dose.

Even so, as vaccinations have picked up across the country, Tal said she’s been getting more questions about whether it was necessary that people return for their second mRNA vaccine dose, especially if they experienced side effects after the first. (Those side effects can range from arm soreness to fever and chills, lasting up to a day or two.)

Her answer is an unequivocal yes.

“The side effects that you face are worth having that strong immune response that is going to protect you from the actual virus,” she said.

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