A doubling of COVID-19 cases in the last two weeks suggests the United States has entered a fourth wave of the pandemic.

No one knows what the next month or two will bring, but the example of the United Kingdom suggests the infection rate could get quite high, while hospitalizations and deaths stay relatively low.

Instead of the virus raging through entire communities, it is expected to target the unvaccinated, including children, and if rates are high enough, also the most vulnerable of the vaccinated – the elderly and the immunocompromised.

"Since the majority of our population is now immune, it's unlikely that we're going to return to the massive nationwide waves we saw back in January," Dr. David Dowdy, an infectious disease epidemiologist with the Hopkins Bloomberg School of Public Health said in a Wednesday webinar with media.

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But major outbreaks can still occur, particularly in areas with low vaccination rates.

"We're going to be living in two pandemic worlds, the world that's vaccinated and the world that's unvaccinated," said Dr. Luis Ostrosky, chief of infectious diseases at UTHealth and an infectious disease specialist at Memorial Hermann-Texas Medical Center in Houston.

The three vaccines authorized for use in the United States, from Moderna, Pfizer-BioNTech and Johnson & Johnson, have all been shown to be highly effective against variants of the virus, including Delta, which now accounts for most of the cases in the U.S.

More than 99% of those currently hospitalized with COVID-19 are unvaccinated. Ostrosky said virtually all his patients are unvaccinated and all regret not getting the shots.

COVID-19 may not be as deadly in this new wave, because older people are largely vaccinated and younger people are less likely to die from an infection, said Ravina Kullar, an infectious disease specialist and epidemiologist and adjunct faculty member at UCLA Medical Center.

But the Delta variant is substantially more contagious than previous ones, though it's still unclear whether it makes people any sicker than previous variants.

"The concern about Delta is well placed," said Dr. Yonatan Grad, an infectious disease specialist at the Harvard T.H. Chan School of Public Health. "We're certainly seeing that this wave is something to contend with and not to take lightly."

COVID-19 rates are rising again

In the U.K., which has roughly the same rate of vaccinations as the U.S., the seven-day average number of infections is back to where it was on Jan. 20, when the country was just a few weeks past its peak.

But hospitalizations there are hovering around 500 a day compared to 4,500 at their January height and deaths remain far lower, with only 26 reported across the country on Tuesday compared to the Jan. 19 peak of over 1,300.

In the U.S. infections have more than doubled since the week of June 22, with total cases rising in 48 states, and deaths are also beginning to climb. Still, the infection rates are 90% below what they were at the January peak.

And there's still another spike expected sometime this fall. The coronavirus that causes COVID-19 is likely a seasonal virus, which means, just as the flu, people are more vulnerable to it in the fall and winter. No one knows when that start date will be, Grad said.

With about 80% of those over-65 fully vaccinated in the U.S., younger people represent a higher percentage of those falling ill. And while children under 12 are unlikely to get a severe case of COVID-19, they are unable to get vaccinated so remain vulnerable to the Delta variant.

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"By virtue of kids not having the opportunity to be vaccinated at the same level as adults, I think they are going to experience a disproportionate burden of infection and sickness from the delta variant," Dowdy said.

The vaccines are good, but not perfect. People who get infected with COVID after vaccination, even if their infection is so mild they don't notice, could be contagious though probably less than those who aren't vaccinated, Grad said.

Three Yankee pitchers – all of whom had been vaccinated – tested positive for COVID-19 on Thursday, forcing the postponement of their first game after the All-Star break.

Those who get mild disease after vaccination could also suffer symptoms of so-called long-haul COVID, said Priya Duggal, an epidemiologist at Johns Hopkins, who was on the call with Dowdy.

People who have caught COVID-19 are also likely to be protected against reinfection for at least a year, according to a study published last month. Researchers found that getting vaccinated after infection boosted by 50-fold the activity of neutralizing antibodies needed to repel the virus and prevented infection with variants.

"There are still unknowns about the extent and duration of protection from natural infection and how well there's protection against new variants," Grad said. "Even people who have had COVID-19 are still advised to get vaccinated."

Although the vaccines appear effective against current variants, if the virus is spiraling out of control anywhere in the world, new variants can arise that could challenge immunity, Dowdy said.

"As long as the virus is circulating, mutating in other countries, it's going to be a threat to us, too," he said.

What can be done?

To reverse the increase in infections, what's needed, "is really injecting a sense of urgency into the equation," Ostrosky said, recommending that people get vaccinated and resume wearing masks indoors when in public.

"If we don't act now, we're just going to be in the same situation we were in a year ago with closures, with disruptions with deaths," he said. "It's very discouraging."

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Ostrosky said he thinks there are two types of people still declining vaccination: those who remain greatly misinformed and those who need more reassurance that they are not going to be harmed by the shots, which have now been given to more than 185 million Americans. "Access is really not the issue right now, it's more reluctance," he said.

Unfortunately, he said, the people who are most reluctant to get vaccinated are also those most reluctant to wear masks.

On Thursday, Los Angeles County announced that it would be reinstituting a mask mandate for indoor public spaces.

Kullar said she wishes the Centers for Disease Control and Prevention had waited longer before saying that masks are unnecessary for the fully vaccinated. Instead of providing an incentive to get vaccinated, the CDC's move simply encouraged everyone, including the unvaccinated, to take off their masks, she said. "It confused the public even more."

She thinks people should continue wearing masks indoors in public places until at least 70% of those in their community or county are vaccinated, "and if you're immunosuppressed, I wouldn't remove your mask."

Outdoors remains safe, she said, particularly if people keep their distance from others.

The one thing that will remain most important in the battle against COVID-19, Ostrosky said, is for people to get vaccinated.

"We can do this," he said. "We have no time to waste."

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