COVID-19 cases are rising sharply again — should we worry?

Here we go again?

COVID-19 cases and hospitalizations are rising once again across the country, driven by more contagious sub-variants of the virus and leaving health experts unsure whether vaccination and immunity from prior infection will be protective enough to prevent yet another deadly wave of infections.

Nearly a third of the country is now registering substantial or high levels of COVID transmission in the last seven days, including most Bay Area counties at the high level, according to the Centers for Disease Control and Prevention.

But now, more than two years into the pandemic, what exactly does that mean in terms of serious illness?

Hospitalizations from COVID are up over 30% in the last two weeks in New York State, said Dr. Céline Gounder, an infectious disease specialist and epidemiologist and Editor-at-Large for Public Health at Kaiser Health News. “The situation in the Northeast may foreshadow what’s to come in the Bay Area.”

Death rates mostly have yet to increase, but in earlier waves, they tended to follow the trend in hospitalizations.

The rising numbers follow a relatively short reprieve from this winter’s nationwide surge propelled by the highly contagious omicron variant. Omicron began spreading in December, with infections peaking a month later and then dropping sharply through February and March.

Nationally, average daily cases are up more than 70% since the end of March, though they remain far below the omicron and delta peaks, and the decline in hospitalization rates appears to be reversing. In California, average daily cases have gone up by more than 50% since the end of March.

The rebound comes just as U.S. officials dropped the mask mandate for public transportation after a judge in Florida said it exceeded their authority — with people from all over now crowding unmasked into planes, buses, subways and rail cars. Though the federal government is appealing the judge’s ruling, airlines and many transit operators indicated the mask mandate isn’t likely to return soon.

New CDC data Thursday showed that the highly transmissible omicron variant that dominated in January has given way to its more contagious cousin, BA.2, which now accounts for three out of four cases across the country. And BA.2 is yielding to an even faster-spreading sister, BA.2.12.1, now one in five cases.

Wastewater surveillance nationally and in the Bay Area tells a similar story, with two-thirds of wastewater sites across the country reporting increases in virus levels.

Experts are watching warily.

The CDC refocused its community-level assessment in February to prioritize how the virus is affecting local hospital capacity over case levels. That new measure shows 94% of U.S. counties still at the green low level.

But even that has changed over the past week for New York, where many counties are listed at the red high level at which the CDC recommends wearing masks in public indoors. Deaths in the Empire State also have begun to tick up, although the rate remains low.

Though the BA.2 omicron subvariant drove sharp increases in cases and hospitalizations in Europe and Asia, health experts remain unsure to what extent that will happen here in the U.S., where two-thirds of Americans have been vaccinated, 45% of those had a booster and many have recovered from recent COVID-19 infection.

In a presentation this past week to the Santa Clara County Board of Supervisors, county Health Officer Dr. Sara Cody said BA.2 is driving “what’s looking like now the beginning of a swell — we certainly hope that it doesn’t take off.”

But Cody added that “it looks like we have a pretty good immunity wall here in our county and in the Bay Area, both in vaccinations and through all the infections from omicron that just ripped through populations.”

“I think that’s why this new BA.2, even though it’s more infectious, was not able to gain a foothold the way that omicron was,” Cody said. But she cautioned that “we should be nothing if not humble two years in. … We just have to do what we always do and watch the trends carefully and see what unfolds.”

Ali H. Mokdad of the University of Washington’s Institute of Health Metrics and Evaluation, which publishes closely monitored COVID-19 projections, said the latest earlier this month foresees a jump in cases that will peak in less than a month.

“We project that we’ll have a bump in cases, but it will be short-lived,” said Mokdad, who formerly worked for the CDC. But he said that with newer mutations of the virus already emerging overseas, it’s no time to be cavalier about the virus and the U.S. should bolster efforts to monitor cases and make treatments available.

“COVID-19 is not done with us,” Mokdad said. “It’s just waiting for us to make a mistake.”

Gounder said “it’s too early to say how worried people should be about the BA.2 and BA2.12.1 variants.” Whether rising infections will progress to severe disease, hospitalization and death, she said, will depend on vaccination rates, particularly among the elderly and other vulnerable populations.

Dr. John Swartzberg, professor emeritus at UC Berkeley’s School of Public Health, noted that while BA.2.12.1 is more transmissible than even BA.2, it does not appear to be more virulent. Even so, he said it’s not a good time for travelers and commuters to be ditching masks on planes, buses and rail cars. Los Angeles County health officials decided to maintain the requirement for local airports and public transit.

“Withdrawing a valuable nonpharmaceutical tool in the face of rising numbers of infections where nobody knows whether it’s going to be just a blip or something profound just seems completely inappropriate,” Swartzberg said.

Cody noted to the county board that “with each generation, the virus seems to become more infectious,” and that even if it’s not more virulent, that can lead to more hospitalization and deaths. A Bay Area News Group analysis found that the more contagious omicron variant killed more Californians at its peak than last summer’s virulent delta variant, including more who were vaccinated and boosted.

“That infectiousness is what’s driving the case rates,” Cody said. “And so even if on an individual level, you’re less likely to have severe disease, on a population level, that can overwhelm a hospital, just because of the sheer number of people getting sick. A small proportion of a large number can still be a large number. And that’s what we saw with our omicron surge.”

Staff writer Harriet Rowan contributed.