1 dose of Moderna, Pfizer vaccines effective against COVID-19. 'Get the second dose,' experts say.

·4 min read

For months, data on the effectiveness of COVID-19 vaccines has been based on information from clinical trials. Now there is new, real-world data out about the effectiveness of the Pfizer-BioNTech and Moderna vaccines — and it's very promising.

New research from the Centers for Disease Control and Prevention released on Monday found that both Pfizer's and Moderna's mRNA COVID-19 vaccines are exceedingly effective at preventing the disease after just one dose, at rates higher than what was previously reported.

The study analyzed data from 3,950 health care personnel, first responders and other essential and frontline workers who underwent weekly testing for SARS-CoV-2, the virus that causes COVID-19, for 13 weeks after they were vaccinated. The researchers discovered that the vaccines were 80 percent effective at preventing COVID-19 after one dose, and 90 percent effective after two doses.

That's slightly different data from what the clinical trials found. In those trials, the Pfizer-BioNTech vaccine was 52 percent effective after one dose and 95 percent effective after two doses. For the Moderna vaccine clinical trials, that vaccine was 50.8 percent effective up to two weeks after the original dose (and 92.1 percent effective after that), with a 94.1 percent efficacy after two doses.

Vials for the Moderna and Pfizer COVID-19 vaccines are displayed on a tray at a clinic set up by the New Hampshire National Guard in the parking lot of Exeter, N.H., High School, Thursday, Feb. 25, 2021, in Exeter. The temporary facility, operating out of a field hospital tent, administers both the Moderna and Pfizer COVID-19 vaccines. (AP Photo/Charles Krupa)
New CDC research found that both Pfizer's and Moderna's mRNA COVID-19 vaccines are very effective at preventing the disease after just one dose. (Charles Krupa/AP)

Why the difference? It's data from the real world vs. a clinical trial. "Real-world data is the best way to gauge the impact of a vaccine, infectious disease expert Dr. Amesh A. Adalja, senior scholar at the Johns Hopkins Center for Health Security, tells Yahoo Life. "These are important findings, showing the value of the vaccine."

Dr. William Schaffner, an infectious disease specialist and professor at the Vanderbilt University School of Medicine, tells Yahoo Life that he's "not entirely sure" why there is such a huge increase in how efficient the vaccines are at preventing COVID-19 after one dose according to real-world data. But, he says, "this is data from continuing analysis of larger populations — and it's encouraging."

The data "supports what many of us have felt," Dr. Shobha Swaminathan, associate professor in the Division of Infectious Diseases at Rutgers, tells Yahoo Life. "After two weeks, most vaccines will have some efficacy, and the degree of efficacy would relate to the potency of the vaccine," she adds.

This study also sampled people "regardless of symptoms" and "may have picked up some asymptomatic cases as well," Swaminathan says.

Adalja says the findings may even end up influencing public policy about who gets vaccinated when. "For now, I think a two-dose regimen will be in place, but this result underscores the importance of prioritizing first doses," he says. "Second doses can be given, but first doses should take precedence."

Schaffner stresses that people shouldn't look at these findings and assume they don’t need a second dose of the COVID-19 vaccine. "Don't try to slice this salami too thin," he says. "I do think we need that second dose in order to get maximum protection." Schaffner points out that people get "much more antibodies" against SARS-CoV-2 with the second dose, as well as "more complete protection" that can help against variants of the virus.

Dr. Thomas Russo, professor and chief of infectious disease at the University at Buffalo in New York, agrees. "I can't emphasize enough how important it is to get the second dose," he tells Yahoo Life. Russo points out that some people are nervous about getting the second dose after hearing that side effects can be worse afterward, but he says that many stories about the side effects have "a little bit of embellishment."

"Only a minority of people have severe, systemic symptoms," he says. "Even if you have flu-like symptoms, you take a 'vaccine day.' It's a small price to pay for getting a really tremendous degree of protection from this coronavirus."

If you need to wait beyond the recommended three to four weeks between vaccinations, that's OK, Schaffner says— as long as you actually complete the regimen. "Please get the second dose. You need to get complete protection," he says. "We can’t tell you that you’re going to have 95 percent protection on the basis of a single dose, and you're likely to have longer protection this way."

Swaminathan agrees. "We do not know how durable the vaccine response will be for these products if people stop at one dose," she says.


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