Amid warning of tough days ahead with surging Covid-19 infections, the director of the National Institutes of Health said Sunday the US could decide in the next couple of weeks whether to offer Covid booster shots to Americans this fall.

Among the first to receive them could be health care workers, nursing home residents and other older Americans.

Dr Francis Collins also pleaded anew for unvaccinated people to get their shots, calling them "sitting ducks" for a delta variant that is ravaging the country and showing little sign of letting up.

"This is going very steeply upward with no signs of having peaked out," he said.

Federal health officials have been actively looking at whether extra shots for the vaccinated may be needed as early as this fall, reviewing case numbers in the US "almost daily" as well as the situation in other countries, where preliminary studies suggest the vaccine's protection against serious illness dropped among those vaccinated in January.

No US decision has been made because cases here so far still indicate that people remain highly protected from Covid-19, including the delta variant, after receiving the two-dose Pfizer or Moderna regimen or the one-shot Johnson & Johnson vaccine.

But US health officials made clear Sunday they are preparing for the possibility that the time for boosters may come sooner than later.

READ: Pfizer to seek OK for 3rd vaccine dose; shots still protect

"There is a concern that the vaccine may start to wane in its effectiveness," Francis said. "And delta is a nasty one for us to try to deal with. The combination of those two means we may need boosters, maybe beginning first with health care providers, as well as people in nursing homes, and then gradually moving forward" with others, such as older Americans who were among the first to get vaccinations after they became available late last year.

He said because the delta variant only started hitting the US hard in July, the "next couple of weeks" of case data will help the US make a decision.

Moderna President Stephen Hoge said seeing some "breakthrough" infections emerge among the vaccinated within six months has been surprising, even if most symptoms so far have not been life-threatening. "I think that suggests we are going to need booster vaccines to get through the winter."

Last week, the Food and Drug Administration said people with weakened immune systems can get an extra dose of the Pfizer or Moderna vaccines to better protect them as the delta variant continues to surge.

"If it turns out as the data come in, we see we do need to give an additional dose to people in nursing homes, actually, or people who are elderly, we will be prepared to do that very quickly," said Dr Anthony Fauci, President Joe Biden's chief medical adviser.

The US is now seeing an average of about 129,000 new infections a day – a 700% increase from the beginning of July – that number could jump in the next couple weeks to 200,000, a level not seen since among the pandemic's worst days in January and February, Francis said.

Both he and Fauci stressed that the best way to stem the virus is for the unvaccinated to get their shots.

Currently, about 60% of the US population has gotten at least one dose and nearly 51% are fully vaccinated, according to the Centers for Disease Control and Prevention. Areas with low vaccination rates have been particularly hit hard with infections, such as Louisiana, Texas, Florida and Mississippi.

The rapidly escalating surge in infections across the US has caused a shortage of intensive care unit beds, nurses and other front-line staff in virus hotspots that can no longer keep up with the flood of unvaccinated patients.

READ: Pfizer studying effects of 3rd vaccine dose

Health officials also warn that more children who are not yet eligible for vaccines could get infected, though it is not clear whether the delta variant leads to more severe illness among them.