Dr Anthony Fauci, the director of the National Institute of Allergy and Infectious Diseases, during a White House press briefing on January 21. Alex Wong/Getty Images

Dr. Anthony Fauci warned on Monday against delaying second COVID-19 shots.

He said it could be a recipe for new variants to spread by giving people weaker immunity.

While one shot gives “some” protection, data suggests two shots are “tenfold” better, Fauci said.

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Dr. Anthony Fauci said making sure people in the US get their second dose of a Pfizer or Moderna COVID-19 vaccine on time could be critically important to curbing the rise of new, fast-spreading virus variants.

Fauci, the director of the National Institute of Allergy and Infectious Diseases, said while it was true that a single shot could “give some degree of protection” to people, it appeared nowhere near as strong as the powerful illness shield that two shots provides.

That’s why he’s not recommending giving more Americans first doses of vaccines and waiting until later on in the year to give people their second “booster” shot, a strategy that’s being tried out now in the UK.

That plan, though it could prove successful at partially protecting more people more quickly, is just too risky and not backed up by any of the data we have so far, he said.

“The way viruses respond to pressure, you could actually be inadvertently selecting for more mutants,” Fauci said Monday during a White House coronavirus briefing. “For that reason, we have continued to go by the fact that we feel the optimum approach would be to continue with getting as many people on their first dose as possible but also making sure that people – on time – get their second dose.”

2 shots provide stronger protection against all forms of the virus, including variantsFauci has already had both of his shots. Patrick Semansky-Pool/Getty Images

Vaccine trials on tens of thousands of volunteers around the world have shown how well authorized vaccines from Pfizer and Moderna work as a two-shot course when they are administered 21 days apart (for Pfizer) or 28 days apart (for Moderna).

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Given in that way, those vaccines appear well over 90% effective at ending symptomatic coronavirus infections. (AstraZeneca’s two-shot vaccine, which is authorized for use in the UK but not yet in the US, appears less effective than both Pfizer’s and Moderna’s.)

Two shots of either Pfizer’s or Moderna’s vaccine, when administered on schedule, can provide “tenfold” the immune protection of one shot, Fauci said.

“The reason that’s important is not only because of the height of the response and the potency of the response, but as you get to that level of antibody, you get a greater breadth of response,” Fauci added.

That “breadth” of virus response could be critically important to help curb the pandemic, especially right now.

“By breadth of response we mean it covers not only the wild type and currently circulating virus, but also the variants that we see circulating, particularly the 1.1.7 and the 3.5.1,” Fauci added, referring to the variants first identified in the UK and South Africa, respectively.

In other words, because vaccines don’t work quite as well against the new variants, it’s important to make sure the coverage we get from them is as beefy as possible, and the way to do that is by giving people two full shots.

It’s looking increasingly likely that the B.1.1.7 variant will become the dominant strain of the virus being passed around in the US by March, Fauci said – a timeline that lines up with other experts’ recent predictions.

It’s still unclear if we could ‘get away with a single dose’A nurse prepares a coronavirus vaccine developed by the National Institutes of Health and Moderna Inc. Hans Pennink/AP

Fauci said it was still “not unreasonable” to wonder whether people might be able to “get away with a single dose” of vaccines from Pfizer or Moderna, at least for a little while.

But he stressed that there were no finalized studies on the effectiveness of that one-shot strategy. By the time rigorous testing on the question could be completed, there would be a better supply of vaccines to go around across the US anyway, making it a bit of a “moot point” for studying, he said.

The strategy could also be a dangerous one.

One shot of Moderna’s or Pfizer’s vaccine will provide a lower degree of infection-fighting power than two full doses would. As a result, Fauci said, giving people just one dose could turn their vaccines into easier-to-tackle threats for the virus, allowing it to learn how to mutate faster and better and continue infecting more vaccinated people.

The important thing, Fauci said, is to “get as many people vaccinated as quickly as we possibly can” with two shots.

“That’s the best defense against the evolution of variants,” he said.

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