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Places such as Vermont where cold weather has set in are seeing an increase in COVID-19 cases. Spencer Platt/Getty Images
  • The average daily number of new COVID-19 cases has been rising in the United States during the past month.
  • Many of those increases are being seen in states with higher vaccination rates.
  • Experts say the onset of cold weather is a major factor as people start gathering indoors more, allowing the novel coronavirus to circulate easier.
  • They also note that new cases aren’t necessarily the best indicators of the seriousness of the current state of the pandemic. Deaths and hospitalizations should be examined, too.

New COVID-19 cases are rising in a number of U.S. states again, including some with high rates of vaccination.

But weather may have as much to do with the trend than vaccination rates, experts say.

The 7-day daily average of new COVID-19 cases had fallen below 50,000 during the middle of the summer before increasing in August and then decreasing again in early autumn, according to data compiled by the Centers for Disease Control and Prevention (CDC).

The upward trend has now started to reappear.

New daily COVID-19 cases have topped 100,000 three times this past week, with a 7-day average reaching about 94,000 new cases per day by midweek last week.

In addition, 39 states experienced increases in COVID-19 cases during the week that ended Nov. 21, according to data compiled by Reuters.

Among the states with rising caseloads:

  • Missouri: 102 percent increase
  • Connecticut: 85 percent increase
  • Michigan: 65 percent increase
  • Oklahoma: 49 percent increase
  • Massachusetts: 48 percent hike

Some of these states have some of the highest rates of vaccination in the country. More than 70 percent of Massachusetts and Connecticut residents, for example, are fully vaccinated against COVID-19.

Meanwhile, some states with low rates of vaccination, such as Louisiana and Georgia, currently have some of the lowest per capita COVID-19 case rates.

“We went through the same cycle last year with different parts of the country… going up at different times,” Dr. Robert C. Bollinger, a professor of infectious diseases at Johns Hopkins University School of Medicine in Baltimore and a founding member of emocha Health.

a professor of infectious diseases, medicine, public health, and nursing as well as director of the Center for Clinical Global Health Education at Johns Hopkins University School of Medicine in Baltimore, told Healthline.

“There are probably a number of factors contributing to the current spike,” Dr. Karen Edwards, chair of the department of epidemiology and biostatistics at the University of California, Irvine Program in Public Health, told Healthline. “Some of the most likely factors are that as the weather gets colder, more people are gathering indoors and in closer proximity to each other, which facilitates transmission between individuals.”

“There may also be less adherence to mask wearing, good hygiene, and social distancing, which combined with more indoor activities, will increase opportunities for infection, especially among the unvaccinated,” said Edwards.

She noted that there are still significant numbers of unvaccinated people even in states with higher vaccination rates, “and are more likely to be infected, have more severe illness, and contribute to the spikes.”

Dr. Joseph Iser, a fellow of the American College of Preventive Medicine, told Healthline that the current increase in COVID-19 cases is likely to get worse in the coming months.

“If you’re looking at the flu season, it really doesn’t start until the cold of late fall or early winter,” said Iser. “I think the COVID surge is going to look pretty serious. I think it’s going to continue to rise until we get more adults vaccinated, more adults to get boosters, and more kids ages 5 to 11 to get vaccinated.”

As for states that seem to be behind the curve of rising case rates despite low vaccination rates, Iser said, “Give it time.”

“Once the cooler weather sets, we’re going to see an uptick in those places, too,” he predicted.

Waning immunity against COVID-19, among both vaccinated people and those who previously contracted the coronavirus, may also be a contributing factor to the upward trend in cases, according to Bollinger.

“People who were vaccinated more than 6 months ago now need a booster,” Bollinger noted.

The prevalence of the highly infectious Delta variant also plays a role in driving cases upward, said Bollinger.

He said it’s likely that at least 90 percent of Americans will need to be immune to the novel coronavirus before COVID-19 is brought under control.

These concerns were paramount even before the announcement late last week about the emergence of the Omicron variant.

“Vaccination by itself is not enough to stop an infectious disease,” said Bollinger. “With such a high transmission rate, you need other measures like masks and limits on indoor gatherings. It doesn’t take many unvaccinated people to drive rates up when you have a highly infectious disease like this.”

Case rates don’t tell the whole story about the latest spike in infections, according to Iser.

Vaccinated people may be getting so-called “breakthrough infections” that add to the case count, but such cases tend to be milder, whereas unvaccinated people are still far more likely to develop severe COVID-19 illnesses.

“Looking at case rates gives you a sense of transmissions in that community,” said Iser. “But if you want to see the seriousness of illness you need to look at hospitalizations and death rates.”

So, while there may seem to be a contradiction in states with lower vaccination rates reporting lower COVID-19 case rates, “there is not a disconnect between vaccination rates and the rates of hospitalization and death,” Bollinger said.

“Hopefully we’re not going to be in the same place as we were last year in terms of the impact on the healthcare system,” he said.

Yet with the COVID-19 death toll already topping 777,000 in the United States, Bollinger said the prospect of 1 million total deaths from COVID-19 this winter or next spring seems likely “if we don’t really turn things around.”