US officials have expressed optimism that Covid-19 booster shot delivery can start for all adults on 20 September, the goal set by President Joe Biden, as cases continue to rage across the country fueled by the highly transmissible Delta variant.

The officials insist, however, that boosters will not be rolled out without US health agencies’ authorization, leaving open the possibility of delays.

Dr Anthony Fauci, ​​head of the National Institute of Allergy and Infectious Diseases and chief medical adviser to Biden, was asked Sunday on CBS’s Face The Nation whether the 20 September goal remained the planned rollout date.

“In some respects, it is. We were hoping that we would get both the candidates, both products, Moderna and Pfizer, rolled out by the week of the 20th. It is conceivable that we will only have one of them out, but the other one will follow soon thereafter,” Fauci said. Pfizer has submitted its data, making it likely to meet this goal, Fauci said; Moderna announced that it has started submitting data.

“The bottom line is, very likely at least part of the plan will be implemented, but ultimately the entire plan will be.”

“We’re not going to do anything unless it gets the appropriate FDA regulatory approval, and then the recommendation from the [CDC] advisory committee,” Fauci also said, explaining that he expects any possible delay with Moderna would be “at most” a few weeks.

As almost all Covid-19 infections in the US are caused by the Delta variant, officials hope boosters will clamp down on its rapid spread. Covid-19 vaccines do provide incredibly strong protection against illness, hospitalization, and death against Delta, but breakthrough infections are reportedly rising with this variant.

At present, 53% of the US population is fully vaccinated, and 62% have received at least one dose.

Covid-19 cases have increased 6% in the past week on 4 September, and there has been a 22% increase in deaths over that same period. The seven-day average for cases and deaths over this same period is 163,716 and 1,550, respectively.

The US continues to lead the world in Covid-19 cases and deaths, at 39,908,072 confirmed infections and 648,121 known fatalities, according to Johns Hopkins University data. Nearly 95% of US counties have “high” community transmission, according to the US Centers for Disease Control and Prevention.

Fauci’s statements come amid questions on Biden’s plans for distributing Covid-19 booster shots. Leaders of the CDC and FDA have implored Biden to reconsider his plan to start offering boosters on 20 September, saying they needed more data, NPR reported.

White House chief of staff Ron Klain echoed Fauci’s statements Sunday on CNN’s State of The Union, saying that 20 September was a projection, not a hard-and-fast date. Klain said that Biden’s discussion of booster implementation had always depended on FDA and CDC authorization.

“I think what we said was that we would be ready as of the 20th,” Klain said. “I would be absolutely clear, no one’s going to get boosters until the FDA says they’re approved, until the CDC advisory committee makes a recommendation.”

“What we want to do though is be ready as soon as that comes.”

Klain also said that the recipients would be determined by FDA and CDC’s scientific guidance.

As discussion of booster rollout continues, public health officials and experts have recently expressed concern that Labor Day holiday travel this weekend could worsen the ongoing surge.

“As we head into Labor Day, we should all be concerned about history repeating itself. High or intense transmission around most of the country combined with population mobility with limited masking and social distancing has been a consistent predictor of major surges,” Dr John Brownstein, a Boston Children’s Hospital epidemiologist, told ABC News.

Data show that holidays can spur dramatic Covid-19 transmission throughout the country. In the weeks preceding Labor Day 2020, average US daily cases dropped to about 38,000. There was a 400 percent increase in daily US cases between Labor Day weekend and Thanksgiving of 2020, however, resulting in record high deaths and hospitalizations, ABC News said.

Dr Rochelle Walensky, Centers for Disease Control and Prevention(CDC) director, said Tuesday during the White House Covid-19 briefing: “First and foremost, if you are unvaccinated, we would recommend not traveling.”

“Throughout the pandemic, we have seen that the vast majority of transmission takes place among unvaccinated people in closed, indoor settings,” Walensky also said.

Jeff Zients, White House Covid-19 response coordinator, similarly commented during this briefing: “We need more individuals to step up, as people across the country prepare for Labor Day weekend. It’s critical that being vaccinated is part of their pre-holiday checklist.”