For the first time in two months, COVID-19 case counts and deaths are rising again in most U.S. states, especially in the northern tier and Rocky Mountains, according to a USA TODAY analysis of Johns Hopkins University data.

Does this mean a fifth wave of the pandemic is cresting?

With a virus this new to humanity it's impossible to say, said Dr. Susan Kline, a professor of infectious diseases at the University of Minnesota medical school. 

"It is just too early to predict this right now. This is only the second year we have experienced such a fall wave," she said.

Experts say the surge is being driven by a combination of factors: the seasonality of the virus, waning immunity and many still unvaccinated Americans. And it underscores the likelihood COVID-19 is here to stay. 

"We saw a very similar trend last year. People are moving indoors now because the weather's getting cold," said Kline. 

In the week ending Wednesday, case counts were higher in 29 states than they were the week before. A month ago, cases were rising in just 12 states.

Hospitalizations also are up. Compared to a week earlier, hospitals in 23 states reported a rising number of likely COVID-19 patients; hospitals in 19 states reported more patients in the intensive care units.

SARS-CoV-2, the virus that causes COVID-19, is behaving like other coronaviruses that have circulated the globe for decades, said Dr. Arnold Monto, a professor of epidemiology at the University of Michigan School of Public Health. 

Four coronaviruses cause the common cold and mild to moderate illness in almost everyone who gets them. InMarch of 2020Monto published a major study looking at when they usuallypeak, coincidentally just as the COVID-19 pandemic was taking off in the U.S. 

"We looked at the data and we said, 'Oh my God, look at the seasonality! It's year after year after year," he said. "It was so dramatic, we couldn't believe it."

These common cold coronavirus illnesses were sharply seasonal, beginning in November and almost disappearing by May, only to return again the following year. 

Monto is betting SARS-CoV-2 behaves the same way. Although the cold-inducing coronaviruses and SARS-CoV-2 are not identical, experts say it's reasonable some of the same seasonal fluctuations should be seen with both. 

Wintertime COVID-19 surges may simply be something we'll have to learn to live with.

"It's not going to go away," said Dr. Timothy Brewer, a professor of medicine and epidemiology at the University of California, Los Angeles. 

Waning immunity

Another reason for the rise could be waning immunity, as those who either got COVID-19 or were vaccinated against it see their protection diminish over time.

"People who had the vaccine last January and February, they're now over six months out," Kline said.

Especially for people with weaker immune systems, which includes the elderly, revaccination is important. 

"We’re in a race between getting people boosted and waning immunity, so boosting is hugely important," said Dr. George Rutherford, a professor of epidemiology and biostatistics at the University of California, San Francisco.

Vermont, for example, has one of the highest vaccination rates in the nation and is experiencing a surge. That's in part because the state began vaccinating its oldest residents first, so immunity among the highest-risk people in the state is likely waning.

"As one of the oldest states, the percentage of Vermonters in this situation is higher than most other parts of the country," Vermont Health Commissioner Dr. Mark Levine said during a news conference Tuesday.

But how long will those second and third doses last? It's an open question with no data available yet.

"We still have much to learn," Kline said. "We need to see how long the immunity appears to last after this year's round of booster shots." 

For those relying on natural immunity, the immune response that comes from having had COVID-19 also appears to wane over time, though the exact timing isn't clear. 

A Danish study found that of nearly 12,000 people who tested positive during the first wave of coronavirus infections in 2020, more than 80% were protected in the second surge. But among those 65 and older, protection against repeat infection was only 47%.

"I’m less impressed with natural immunity holding the line for a long time," Brewer said. 

More: Natural immunity is good. Getting vaccinated after being sick with COVID-19 is better.

Danger greatest for unvaccinated

Vaccination is critical to making COVID-19 a disease we can live with rather than a disease that kills, experts stress.

"If our population were 100% vaccinated we'd be having a very different conversation," Rutherford said.

But case rates are rising both in states with low levels of vaccination, such as Wyoming (45%) and Idaho (56%) and in states with high levels, such as Vermont (72%) and Minnesota (62%).

How is that possible?

"The thing to remember is that if you have 62% or 72% of your population fully vaccinated, that means you 38% or 28% who aren't – and that's plenty of people to sustain transmission of the virus," Rutherford said.  

Tracking COVID-19 vaccine distribution by state: How many people have been vaccinated in the US?

Rising hospitalization rates also don't tell the full story because the unvaccinated are most likely to get sick, to be hospitalized and to die.

In Minnesota, about 2% of people vaccinated against COVID-19 have had a breakthrough infection. But breakthrough infection is not the same thing as breakthrough disease.

Among vaccinated people who had COVID-19, just 0.09% were hospitalized and just 0.015% died, according to the Minnesota Department of Health.

The same holds true in California. "Your chance of being hospitalized for COVID-19 is about 10 times higher if you're unvaccinated and it's 21 times higher to die," Brewer said.

In it for the long haul

This yearly waxing and waning of COVID-19 is likely what the future looks like, experts say. 

"In some years rates will be higher, in some lower. But we're never going to get back to zero cases," Brewer said. 

New babies are always being born whose immune systems have never been exposed to SARS-CoV-2, immunity either through infection or vaccination wanes over time and as people age their immune systems become less able to mount a strong response to the virus.

With the coronaviruses that cause the common cold, the median time between reinfections is about three years, Monto said. 

"We need to start thinking about vaccination as a long-term activity," he said.

Some have suggested a three-dose series of COVID-19 vaccine might provide lifetime immunity, but Monto disagrees. Instead, it will be more like the flu, he said, where every year a new, updated vaccine is offered to keep up with the mutating virus.

"I suggest we're going to get into the same situation," he said, "with COVID vaccination, we're going to have to do it regularly." 

Contributing: Mike Stucka

This article originally appeared on USA TODAY: COVID-19 case rates rise as vaccine immunity wanes and seasons change

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