The main reason for boosters, Fauci said in a telephone interview, is to reverse the trend of rising "breakthrough" infections among people who are fully vaccinated, a point that many experts dispute.
Available data has shown that most severe breakthrough cases have occurred in people over 65 or among those who are immunocompromised. That latter group is already recommended for a third dose.
Dr. Larry Corey, a virologist at Seattle's Fred Hutchinson Cancer Center who is overseeing the U.S.-government backed COVID-19 vaccine trials, is a proponent of using booster doses to bolster antibody levels enough to prevent infection.
"If you don't get infected, you're not going to transmit it to others, and we will more effectively abort the epidemic, and that has economic benefits," Corey said.
The problem there, many experts point out, is that there is scant scientific evidence showing that boosters will in fact prevent infections and transmission.
Some government studies have shown that when infected with the Delta variant, fully vaccinated people can transmit the virus, mostly to unvaccinated people.
"If you take a look at the evidence from the United States, it is very clear that protection against infection and mildly to moderately symptomatic disease is diminishing," Fauci said.
It is happening among many of the U.S. study populations – including a recent study of 600,000 COVID-19 cases in 13 states and large cities. “Not dramatically, but enough," he said.
'WHAT'S THE GOAL'
While some 63% of eligible people in the United States are fully vaccinated, the Delta variant has caused a surge in the deadly disease among the unvaccinated.
The two most widely used vaccines in the United States – those based on messenger RNA (mRNA) technology made by Pfizer Inc PFE.N with BioNTech SE BNTX.O and Moderna Inc– are highly effective but less so against Delta. Cases are rising among the vaccinated - including some that result in hospitalization and death.
The U.S. Food and Drug Administration will release data on a possible booster shot on Wednesday morning ahead of a Friday meeting of its vaccine advisory panel. It will include a briefing on the impact of boosters in Israel, where the government has closely tracked recipients of the Pfizer/BioNTech shot.
During weekly White House COVID-19 briefings, U.S. Centers for Disease Control and Prevention (CDC) Director Dr. Rochelle Walensky, White House COVID-19 Response Coordinator Jeffrey Zients and Fauci have expressed concern that waning immunity against mild, symptomatic COVID-19 could lead to reduced protection against severe disease, hospitalization and death, pointing to data from Israel.
Other countries that have started or are planning booster programs, including Israel - and in the U.K. for people over 50 - have been more frank about the goal to reduce transmission.
Debate over booster doses in the United States has become a sore point for virologists, who largely remain unconvinced that the vaccines are losing their ability to prevent severe disease and hospitalization.
This week, an article in the journal Lancet by two departing FDA vaccine experts and senior scientists at the World Health Organization, challenged the rationale for booster doses, saying more evidence is needed to justify their widespread use and that most cases of COVID-19 are spread by the unvaccinated.
Dr. Paul Offit, an infectious disease expert at the University of Pennsylvania and a member of the FDA' vaccine advisory panel, is unconvinced by arguments that boosters are needed just yet.
“The question is, what's the goal If the goal of the third dose is to enhance protection against serious illness, there's no evidence that that's a problem," Offit said. If the goal is to increase levels of neutralizing antibodies with the aim of decreasing asymptomatic or mild cases, “then we should see those data.”
Corey said the bar for proving a vaccine prevents disease transmission is high.
"Is there proof today No, but there is every reason to believe that this is possible, and may be beneficial," he said.
Fauci, however, said Israeli data shows that since the booster campaign started, the country has started seeing a decline in the virus’ reproduction number, which represents how many other people a person will COVID-19 is likely to infect. The more immunity in a population, the lower the reproduction number.
Fauci said he is confounded by arguments from vaccine experts that boosters are only needed when vaccines stop preventing severe disease, hospitalization and death.
“What is the magic, mystical issue about hospitalization I don’t understand that," he said. "What we’re really saying is we don’t care about anything except keeping people out of the hospital. Really You’re kidding.”
(Reporting by Julie Steenhuysen; Additional reporting by Nandita Bose in Washingtong; Editing by Caroline Humer and Bill Berkrot) ((email@example.com; 312-408-8131; Reuters Messaging: Julie.firstname.lastname@example.org))