WASHINGTON (NYTIMES) - A wedding in Oklahoma leads to 15 vaccinated guests becoming infected with the coronavirus. Raucous Fourth of July celebrations disperse the virus from Provincetown, Massachusetts, to dozens of places across the country, sometimes carried by fully vaccinated celebrants.

As the Delta variant surges across the United States, reports of infections in vaccinated people have become increasingly frequent - including, most recently, among at least six Texas Democrats, a White House aide and an aide to House Speaker Nancy Pelosi.

The highly contagious variant, combined with a lagging vaccination campaign and the near absence of preventive restrictions, is fuelling a rapid rise in cases in all states, and hospitalisations in nearly all of them. It now accounts for about 83 per cent of infections diagnosed in the United States.

But as worrying as the trend may seem, breakthrough infections - those occurring in vaccinated people - are still relatively uncommon, experts said, and those that cause serious illness, hospitalisation or death even more so. More than 97 per cent of people hospitalised for Covid-19 are unvaccinated.

"The takeaway message remains, if you're vaccinated, you are protected," said Dr Celine Gounder, an infectious disease specialist at Bellevue Hospital Center in New York. "You are not going to end up with severe disease, hospitalisation or death."

Reports of breakthrough infections should not be taken to mean that the vaccines do not work, Dr Anthony Fauci, the Biden administration's top pandemic adviser, said on Thursday (July 22) at a news briefing.

"By no means does that mean that you're dealing with an unsuccessful vaccine," he said. "The success of the vaccine is based on the prevention of illness."

Still, vaccinated people can come down with infections, overwhelmingly asymptomatic or mild. That may come as a surprise to many vaccinated Americans, who often assume that they are completely shielded from the virus. And breakthrough infections raise the possibility, as yet unresolved, that vaccinated people may spread the virus to others.

Given the upwelling of virus across much of the country, some scientists say it is time for vaccinated people to consider wearing masks indoors and in crowded spaces like shopping malls or concert halls - a recommendation that goes beyond current guidelines from the Centres for Disease Control and Prevention, which recommends masking only for unvaccinated people.

The agency does not plan to change its guidelines unless there is a significant change in the science, said a federal official speaking on condition of anonymity because he was not authorised to speak on the matter.

The agency's guidance already gives local leaders latitude to adjust their policies based on rates of transmission in their communities, he added. Citing the rise of the delta variant, health officials in several California jurisdictions are already urging a return to indoor masking; Los Angeles County is requiring it.

"Seat belts reduce risk, but we still need to drive carefully," said Dr Scott Dryden-Peterson, an infectious disease physician and public health researcher at Brigham & Women's Hospital in Boston. "We're still trying to figure out what is 'drive carefully' in the delta era, and what we should be doing."

The uncertainty about Delta results in part from how it differs from previous versions of the coronavirus. Although its mode of transmission is the same - it is inhaled, usually in indoor spaces - delta is thought to be about twice as contagious as the original virus.

Significantly, early evidence also suggests that people infected with the delta variant may carry roughly a thousandfold more virus than those infected with the original virus. While that does not seem to mean that they get sicker, it does probably mean that they are more contagious and for longer.

For the average vaccinated person, a breakthrough infection is likely to be inconsequential, causing few to no symptoms. But there is concern among scientists that a few vaccinated people who become infected may go on to develop long Covid, a poorly understood constellation of symptoms that persists after the active infection is resolved.

Much has been made of delta's ability to sidestep immune defences. In fact, all of the existing vaccines seem able to prevent serious illness and death from the variant. In laboratory studies, Delta actually has proved to be a milder threat than Beta, the variant first identified in South Africa.

Whether a vaccinated person ever becomes infected may depend on how high antibodies spiked after vaccination, how potent those antibodies are against the variant, and whether the level of antibodies in the person's blood has waned since immunisation.

In any case, immune defences primed by the vaccines should recognise the virus soon after infection and destroy it before significant damage occurs.

"That is what explains why people do get infected and why people don't get seriously ill," said Michel C. Nussenzweig, an immunologist at Rockefeller University in New York. "It's nearly unavoidable, unless you're going to give people very frequent boosters."

There is limited evidence beyond anecdotal reports to indicate whether breakthrough infections with the delta variant are more common or more likely to fan out to other people. The CDC has recorded about 5,500 hospitalisations and deaths in vaccinated people, but it is not tracking milder breakthrough infections.

Elyse Freitas was shocked to discover that 15 vaccinated people became infected at her wedding. Freitas, 34, a biologist at the University of Oklahoma, said she had been very cautious throughout the pandemic, and had already postponed her wedding once. But after much deliberation, she celebrated the wedding indoors on July 10.

Based on the symptoms, Freitas believes that the initial infection was at a bachelorette party two days before the wedding, when a dozen vaccinated people went unmasked to bars in downtown Oklahoma City; seven of them later tested positive. Eventually, 17 guests at the wedding became infected, nearly all with mild symptoms.

"In hindsight, I should have paid more attention to the vaccination rates in Oklahoma and the emergence of the delta variant and adjusted my plans accordingly," she said.

Health officials should also help the public understand that vaccines are doing what they are supposed to - preventing people from getting seriously ill, said Kristen Panthagani, a geneticist at Baylor College of Medicine who runs a blog explaining complex scientific concepts.

"Vaccine efficacy isn't 100% - it never is," she said. "We shouldn't expect Covid vaccines to be perfect, either. That's too high an expectation."