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WASHINGTON — Summer is coming, and Dr. Anthony Fauci is hopeful that a nation devastated by the coronavirus pandemic might soon be within grasp of some normalcy.

But reflecting on the anniversary of the pandemic that prompted widespread lockdowns across the United States, Fauci also expressed caution, sharing the most important lesson he has learned in the past year: "Don't ever underestimate this virus."

The leading U.S. infectious disease expert and President Joe Biden's chief medical adviser told McClatchy in an interview Wednesday that, with more than 2 million Americans getting vaccinated every day, the country could reach a level of protection within months that would begin lowering the risk of individuals contracting the virus in social settings.

"We are on track to have enough vaccine to vaccinate every adult in this country by the end of May," Fauci said. "If we successfully implement that, and people come forward and get vaccinated, and we don't get stymied by the appearance of variants that interfere with the pace of that plan, then as we start approaching April, May, June and the summer, we should see a considerable degree of flexibility in what we can do — much more than what we can do right now."

The Biden administration has doubled the vaccine supply being sent to states each week since taking office. Three vaccines are available to Americans on an emergency basis, produced by Pfizer, Moderna and Johnson & Johnson.

Fauci and other Biden administration officials are frequently asked when they expect the nation to reach a level of collective protection, sometimes referred to as "herd immunity." Fauci described the country's pathway to herd immunity more like a sliding scale, with public health guidelines becoming increasingly flexible as a greater percentage of the population is vaccinated.

He referred to Israel as an example of a country that has begun to loosen its restrictions as more of its citizens have been vaccinated.

"I think if we continue to push with the vaccinations and we get 2 million or more people vaccinated every day, by the time we get to the end of the spring and the beginning of the summer, I think we're going to have considerably more leeway in doing things that approach normality," Fauci said.

"I think you could feel comfortable, unless something happens like another surge, which I hope and doubt will occur, but you never can rule that out — that's a possibility," he said.

The Centers for Disease Control and Prevention released new guidance this week on what activities are safe to resume after individuals are fully vaccinated. The agency said it is safe for vaccinated people to gather with other vaccinated people indoors, or with unvaccinated individuals from a single household if they are at low-risk of contracting a severe case of COVID-19.

Fauci said his unofficial guidance is to keep gatherings of vaccinated people at a "modest" size of no more than 10 people, noting that the CDC did not provide specific numbers.

"The CDC is leaving it up to good judgment — a modest size. So for me, a modest size is anywhere between five and ten people. I think when you get to 30, 40, 50, that's not an intimate gathering," he said.

The longtime director of the National Institute of Allergy and Infectious Diseases tempered his hope with caution, noting how the novel coronavirus that first sparked a pandemic one year ago has consistently surprised scientists.

Whether Americans will enjoy more flexibility in summer will rely, in part, on the spread of several variants of the coronavirus in the country, including two that were first identified in the United States as well as others from Brazil, the United Kingdom and South Africa.

But Fauci expressed optimism that the country could "keep the variants under reasonably good control" and said that a booster shot under development to address the South African variant as a precautionary measure might not even be needed, if enough Americans are protected against the original strain that first took hold in the United States.

"You know, I'm not sure we will even need them," Fauci said. "We haven't made a definitive decision on whether or not we're going to use that. We may not use it, because the South African variant relative to other variants may become less important."

Instead, booster shots against that original strain might be a more efficient way to provide a level of protection against several emerging variants, he said. All available vaccines have shown to provide some degree of protection against the variants that have emerged.