Pfizer vaccine 85 percent effective after first dose, Israeli study finds

The Pfizer COVID-19 vaccine is 85 percent effective 15-28 days after just one dose, according to a new Israeli study, helping bolster the case for possibly delaying the second dose of vaccine to cover more people.

The study of health care workers at Sheba Medical Center in Israel, published in the journal The Lancet, found the vaccine was 85 percent effective in preventing symptomatic cases of COVID-19. When asymptomatic cases were included too, the figure was 75 percent.

Those are promising results for just one dose of the vaccine. The Pfizer clinical trials were conducted with a two-dose regimen, which found 95 percent effectiveness.

Some experts have been pushing for delaying the second dose in an effort to vaccinate more people faster. The United Kingdom has controversially adopted such a strategy.

A limitation of the study, though, is that it does not determine how long the protection from just one dose will last.

Some experts are also worried that providing a weaker vaccination of just one dose could allow dangerous variants of the virus to evolve to evade that lower level of protection. 

“Early reductions of COVID-19 rates provide support of delaying the second dose in countries facing vaccine shortages and scarce resources, so as to allow higher population coverage with a single dose,” the authors write in The Lancet. “Longer follow-up to assess long-term effectiveness of a single dose is needed to inform a second dose delay policy.”

Anthony Fauci, the government's top infectious disease expert, responded to the study at a White House press briefing Friday, saying he still thinks the U.S. should stick with a two-dose approach.

Fauci raised the concerns that it is unclear from the study how long the one-dose protection will last, and noted that the immune response is much stronger after the second dose, which also provides more cushion if the new variants of the virus erode the vaccine's protection somewhat.

"The one thing we don't know is how durable it is," he said of one dose.

"The second dose is ten times higher in the sense of level of neutralizing antibodies," he added.

—Updated at 12:48 p.m.