Seasonal changes in COVID-19 likely about people as much as virus, weather

Minnesota officials hope rising temperatures and outdoor activity will further suppress the spread of the coronavirus that causes COVID-19, but how much the weather affects the pandemic's course is unclear.

While studies have correlated seasonal changes with viral transmission levels — and two COVID-19 waves in Minnesota have both peaked in the spring and bottomed out in the summer — public health experts said weather is unlikely a singular factor.

"Does this virus show seasonality? I don't know that we can say that," said Kris Ehresmann, state infectious disease director. "But the thing about spring and summer — people are able to be outdoors more, and that's a much safer environment."

Viral activity has plummeted in Minnesota after a third pandemic wave emerged in March and peaked in April. While Friday's report of eight COVID-19 deaths and 257 infections raised Minnesota's pandemic totals to 7,445 fatalities and 602,134 infections, all indicators of viral transmission levels in the state declined.

The positivity rate of COVID-19 testing in Minnesota dropped to a record low of 3%. COVID-19 hospitalizations in the state also dropped from a peak of 699 on April 14 to 252 on Thursday.

The first pandemic wave in 2020 similarly declined by summer, but Ehresmann said the circumstances were unique beyond the weather, with Minnesota emerging from a 51-day lockdown that limited face-to-face contact.

The decline in this spring's wave is being largely credited to COVID-19 vaccine.

Even so, several studies have found that weather patterns could predict COVID-19 levels independently of state mitigation strategies. Researchers at the Massachusetts Institute of Technology reported in March that changes in humidity and temperature influenced COVID-19 levels in temperate regions, and that other atmospheric conditions and ultraviolet radiation affected viral transmission everywhere.

Similarly, a study published in April in the journal Scientific Reports estimated that COVID-19 rates increased with each degree in latitude away from the equator, where heat and humidity are higher, and that summer weather supported public health efforts to contain the pandemic.

While this matches with research showing that cooler, drier air allows other respiratory viruses to thrive, at least one U.S. research group concluded that the weather's impact is really on human behavior, which in turn affects COVID-19 risks.

Biostatistics researcher Christina Ramirez at the University of California, Los Angeles, and colleagues found that extreme weather at both ends predicted coronavirus spread in combination with population density.

"When its really cold, most people remain indoors," Ramirez said. "And because its cold, they're not going to open their doors or open their windows, which really helps with ventilation and reduces transmission.

Minnesota Gov. Tim Walz urged vaccination to reduce risks in any climate, but said it will be interesting to see if the pandemic follows last summer's pattern of peaking in the South after declining in the Midwest. Several Southern states have below average vaccination rates.

"We saw sort of an inverse relationship," Walz said. "Southern states peaked in the summer last year. We peaked in the fall. We will see if that continues to happen."

Critics of Minnesota's pandemic response have offered seasonality as a reason infection numbers rise and fall, regardless of government mask mandates or social distancing rules.

However, Michael Osterholm of the University of Minnesota's Center for Infectious Disease Research and Policy said there is too much confounding evidence to conclude that climate plays such an influential role.

Seasonality doesn't explain why COVID-19 activity is peaking in Manitoba but not Saskatchewan, he said, or why the two global hot spots right now are India and Pakistan in Asia, and Uruguay and Paraguay in South America.

Health officials generally agree that SARS-CoV-2 transmission could develop into a seasonal pattern with time — especially if vaccination reduces viral spread but doesn't eliminate it entirely. Four other coronaviruses that cause about one-fourth of the common colds in the U.S. are mostly seasonal.

While seasonality wasn't a big factor in the first pandemic phase when the population wasn't immune to SARS-CoV-2, it could become influential over time as the virus seeks to spread among people who remain vulnerable, said Dr. Ruth Lynfield, Minnesota state epidemiologist.

"As time goes on and your susceptible population is smaller ... then seasonality may have a stronger role," she said.

Even seasonal influenza isn't exclusively seasonal — with outbreaks in summer camps going unnoticed because cases aren't tracked at that time of year, said Dr. Eric Toner, a senior scholar at the Johns Hopkins Center for Health Security at the Bloomberg School of Public Health.

Flu mostly accelerates in the fall and winter because it spreads so easily indoors and among children once they are together in school, he said. SARS-CoV-2 doesn't spread as easily among children as it does among adults but could gain a similar foothold in the fall when people stay indoors.

"The one thing in my view that should change for all of these respiratory viruses is that we should wear masks during cold and flu and COVID season," he said. "We have shown how effective masks are, and so, like we see in other countries, particularly in Asia, if I'm on the subway or an elevator when someone is sniffling and coughing, I'd really like them to wear a mask. Or I would want to wear a mask. Or both, even better."