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Public health experts say there appears to be a seasonality to COVID-19. But the preliminary data suggests that COVID-19 cases increase in the winter, not in the summer.
It is the increased transmissibility of the delta strain that is causing the spike in cases, irrespective of the weather, an expert said.
Masking, social distancing and vaccination can combat the delta variant and bring down cases, experts said.
As the numbers of new COVID-19 cases rise in Florida, Gov. Ron DeSantis has claimed that the ongoing spike is to be expected — that it’s part of a seasonal pattern.
"We have a summer season here, just like last year. It started a little later this year. So you’re going to have higher prevalence for the rest of July, probably into August. And then it goes back and goes the different waves," DeSantis said July 21. "If you’re vaccinated, those waves are not going to impact you in any significant way, and I think that’s the important message for people."
He said something similar about seasonality a couple of days before that.
"Obviously in July, which I told people months ago, we would see higher prevalence because this is a seasonal virus, and this is the seasonal pattern that it follows in the Sun Belt states, particularly in Florida," DeSantis said July 19.
Weekly data reported by the state’s health department shows that new cases have steadily increased in Florida all season: Nearly 12,000 cases were reported in a week in early June, more than 23,000 cases in a week at the beginning of July, and by early August, more than 151,000 were reported in a week.
Critics argue that it’s the governor’s policies that are fueling Florida’s rising cases. He’s banned local governments from setting pandemic restrictions, opposed vaccine mandates and opposed requiring masking indoors.
Some experts said there appears to be a seasonal component to COVID-19, but not the way DeSantis describes it. They said that data suggests that cases, deaths and other epidemiological variables rise during winter, not during summer.
Respiratory viruses, like the influenza, peak in winter, said Jill Roberts, an associate professor at the University of South Florida's college of public health.
"Despite our hot weather and Floridians often retreating to AC in summer, influenza continues to display winter seasonality in Florida (not summer)," she said.
Even if there is a seasonal component to COVID-19, experts said, one should not dismiss the importance of masking, social distancing and vaccination in mitigating the spread and the severity of COVID-19. People who are fully vaccinated against COVID-19 can still get infected with the coronavirus, but the Centers for Disease Control and Prevention says evidence shows that people who are vaccinated will likely have less severe illness.
The delta variant of the coronavirus that causes COVID-19 is currently spreading worldwide and has caused outbreaks regardless of the season, Roberts said.
"It is the increased transmissibility of this strain that is causing the cases, irrespective of the weather," she said.
In the absence of community immunity, seasonality has little to no impact on spread of respiratory viruses — it can happen year-round, Roberts said.
DeSantis doesn’t dispute that COVID-19 can spread year-round, said spokesperson Christina Pushaw. But she said that a seasonal pattern is the "root cause" of the current spike in Florida, while the delta variant has been an accelerant and population immunity from previous infection or vaccination has been an impediment.
"There is some speculation that COVID infections rise as people spend more time indoors in summer in the South, because of the extreme heat at this time of year," she said. "We know COVID spreads much more easily indoors than outdoors. That may also be a reason why we see such serious spikes in northern states in the winter/spring, as in Michigan a few months ago, despite that state’s mandates and restrictions."
But behavior "is not the only or even the main factor influencing COVID prevalence," Pushaw argued. DeSantis "looks at the empirical evidence, which shows that COVID prevalence rises and falls in different regions at different times of year, in ways that cannot be explained by ‘behavior,’" she said.
One study on the seasonality of COVID-19, cited by Pushaw as backup for DeSantis’ statements, highlighted five countries of different climates and latitudes: Canada, Germany, India, Ethiopia and Chile.
In Canada, Germany and Chile, COVID-19 incidence peaked in their respective winter months. While in India and Ethiopia, COVID-19 appeared to have peaked during the summer, "specifically at the time of the monsoons when the specific humidity was at its highest."
The researchers suggested that air drying capacity and ultraviolet radiation, "unlike temperature and humidity, are likely environmental determinants of COVID-19 spread and its seasonality." They said COVID-19 spread appears lowest at high temperatures, and peak spread happens at somewhat average temperatures, "and the relationship is altogether inconsistent."
Researchers said they did not consider the effects of climate control such as heating and air conditioning, "which may also influence the seasonality" of viral respiratory diseases.
Their findings didn’t entirely align with DeSantis’ argument. Overall, researchers said that the spread of COVID-19 cannot be described by environmental factors alone.
"The spread of COVID-19 is largely shaped by several factors like population density, social distancing, international travel routes, school closures, event cancellations, mask mandates and hygiene," said the study, published in May.
Another study cited by Pushaw was published in January and was co-authored by Gustavo Caetano-Anollés, a professor of bioinformatics at the University of Illinois.
Caetano-Anollés told PolitiFact that their study showed a seasonal component to COVID-19 and that additional analysis supported those findings. But it shouldn’t be used to support a claim of a summer spike in Florida, he said, because their data shows that cases go up in colder temperatures, not as it gets hotter.
Caetano-Anollés said he was unaware of a study testing the hypothesis that cases go up due to summer crowding.
In Florida and the U.S. in general, human behaviors "are more important drivers than environment and physiology" and the virus’ winter seasonality will not improve conditions, he said.
"Seasonality expresses during an entire yearly cycle, not during some few months of that cycle," Caetano-Anollés said. "Being seasonal will have little effect on a raging Florida pandemic."
Unless vaccination rates increase and preemptive measures are imposed, cases and deaths will continue to soar, he said.
Experts did not offer a specific timeline. But cases should decrease "as this wave burns itself out" by infecting the maximum number of people, Roberts said.
"This is absolutely not the route of immunity that is desired as it will and is resulting in unnecessary hospitalizations, deaths, and burden to the health care and laboratory systems," she said.
Email interview, Christina Pushaw, spokesperson for Gov. Ron DeSantis, Aug. 11, 16, 2021
Phone and email interview, Peter Katona, Chair, Infection Control Working Group
Clinical Professor of Medicine, at UCLA Schools of Medicine and Public Health, Aug. 12, 2021
Email interview, Gustavo Caetano-Anollés, a professor of bioinformatics at the University of Illinois, Aug. 12, 2021
Email interview, Jill Roberts, an associate professor at the University of South Florida's college of public health, Aug. 16, 2021
Burra P, Soto-Díaz K, Chalen I, Gonzalez-Ricon RJ, Istanto D, Caetano-Anollés G. Temperature and Latitude Correlate with SARS-CoV-2 Epidemiological Variables but not with Genomic Change Worldwide. Evolutionary Bioinformatics. January 2021. doi:10.1177/1176934321989695
WFLA.com, ‘Vaccines are saving lives’: DeSantis stresses importance of shots, criticizes mask mandates as Florida COVID cases spike, July 21, 2021, updated July 23, 2021
TheFloridaChannel.org, courtesy of WFTV, 7/19/21 Ceremonial Signing of SB 976 – Protection of Ecological Systems
Florida Department of Health, COVID-19 weekly Situation Report: State Overview
Florida Gov. Ron DeSantis’ website, Governor Ron DeSantis Signs Landmark Legislation to Ban Vaccine Passports and Stem Government Overreach, May 3, 2021; Executive Order 21-81