Why didn't summer kill the coronavirus? Experts explain what White House officials were missing

UPDATE 7/17/20: In a statement to Yahoo Life, a spokesperson for DHS said that the studies (which have been "peer-reviewed and published in several scientific journals") are just one part of the equation. "As we have always maintained, the impacts of environmental factors — temperature, humidity, and sunlight — only relate to the natural decay of the virus on surfaces and in airborne particles." The spokesperson noted that Bryan mentioned these are "only a couple of the potential ways" the virus can spread and reasserted that preventative measures such as social distancing, mask-wearing and frequent hand washing are "much more effective at preventing or reducing virus transmission."


When the coronavirus pandemic hit the United States in March, White House officials’ hopes seemed high that summer would stem the tide of COVID-19. During an April 24 White House coronavirus task force briefing, William Bryan, the acting undersecretary for science and technology at the Department of Homeland Security, touted “emerging evidence” that reportedly showed SARS-CoV-2 would die more quickly in the presence of sunlight and humidity.

Scientists were dubious that this would have a major effect, pointing to countries with already-warm weather that were then experiencing outbreaks. Now mid-July, with over 3.4 million U.S. cases and record-breaking daily cases in some of the warmest states, scientists are realizing their predictions were right — and are shining a light on what the officials may have missed.

Rachel Baker, a postdoctorate fellow at the Princeton Environmental Institute, was one of three authors of a study published May 15 in the journal Science on how SARS-CoV-2 — the virus that causes COVID-19 — may be affected by climate. In it, she and her colleagues concluded that summer was unlikely to have a major effect on the spread of COVID-19. Their findings did not inherently contradict Bryan’s; they too found that the virus may be weakened by sunlight and warmth. But their research contained a pivotal missing piece: that a virus with zero immunity is more powerful than both.

“When you have a new virus and the population lacks any immunity to that virus, it’s just going to spread really well, and the climate is not going to matter much,” Bakers tells Yahoo Life. “We made that prediction in May and are now seeing all the cases come up in Florida and Texas, these places that are very warm, have lots of UV light, all these kinds of climate variables that people were saying might slow down the virus. But that ended up not being the case. It was sort of sad but interesting to see those predictions come true.”

Baker and her colleagues at Princeton study how these climate variables have an impact on viruses that are already circulating in the population — viruses such as influenza, which has been shown to thrive during the winter months. In the absence of a vaccine (which Baker is hopeful remains on the horizon), SARS-CoV-2 could ultimately play out the same way. “When we run our models into the future two to three years, you do see — without a vaccine — the virus would settle down into kind of a seasonal pattern,” says Baker. “It would look more like the flu. But at this stage, the lack of population immunity is the fundamental driver.”

The group at Princeton was not the only one to study how the virus may act in different seasons. Experts from the University of Maryland’s Institute of Human Virology published a study in mid-June in JAMA suggesting that the SARS-CoV-2 virus may be affected by heat and sunlight (and thus more likely to spread in colder, drier climates). But in an interview with Yahoo Life, Dr. Mohammad Sajadi, an infectious disease researcher in at the University of Maryland School of Medicine and one of the study’s authors, says the conclusion remains the same.

“There is really no immunity toward this virus,” says Sajadi. “Whenever we have a new virus or pandemic they don’t quite act in a manner that you can predict.” This uncharted territory is something he remembers from viruses past, including the H1N1 pandemic in 2009, which peaked in the summer (an unusual development for influenza) before peaking “multiple times bigger” in the fall and winter. “That’s kind of what remains to be seen with this virus,” says Sajadi, “whether we are going to have that kind of peak again in the wintertime.”

Augustin Vintzileos, an assistant research scientist at the University of Maryland’s Earth System Science Interdisciplinary Center and another author of the study, adds that even viruses that are weakened by sunlight and warmth may not disappear in the summer months — something he attributes to the “mobility” factor. “For example, high temperature and humidity in the South make people go indoors where the air circulates through air conditioning,” says Vintzileos. Countless studies, as Vintzileos points out, have shown that the virus is more likely to spread indoors. “What I’m saying is that the apex of climate is much more complicated than we are discussing now,” he adds.

While many questions remain regarding SARS-CoV-2 and how it is affected by climate, both Sajadi and Vintzileos say that looking at temperature, humidity and the change in mobility these cause still aren’t enough. In this current pandemic, the main thing they’ve learned thus far is that humans have the most power to influence the spread of the virus.

Sajadi explains: “For example, the weather conditions were very similar in parts of New Zealand, parts of southeast Australia, South Africa and then also parts of Argentina. But if you look at the cases, they’re very different,” says Sajadi. “There’s raging infections in Argentina and South Africa, almost none in New Zealand and very few in Australia. ... So that points to the fact that the weather’s not everything. The interventions that are undertaken really matter as well.”

Yahoo Life Medical Contributor Dr. Dara Kass agrees that Americans, if taking the proper precautions, could see the benefits of weather on this pandemic. “What we know is that this is a new virus. So there is no background immunity, but more important, we are spreading this virus among one another actively by not washing hands, not socially distancing and not wearing masks,” says Kass. “If we did these things in the summer, we’d have a better chance of getting this virus at bay than in the winter because we do have the heat, the outside, the UV light, which does have some effect on viruses, including this one.”

Kass points out how in many Southern states, where there are higher UV indexes and warmer temperatures, cases are actually increasing at a higher rate. This is the clearest evidence, she says, that simply leaning on the weather to solve this problem is not enough. “It is dangerous if people think the sun alone will save the day; it’s not going to happen,” says Kass. “We need to get everybody back to the same set of facts. It is true that viruses are weakened by the sun or by UV light, but it is also true that that will not be enough if we don’t do our part as citizens of this community.”

The Department of Homeland Security did not respond to Yahoo Life’s request for comment on this story.

For the latest coronavirus news and updates, follow along at https://news.yahoo.com/coronavirus. According to experts, people over 60 and those who are immunocompromised continue to be the most at risk. If you have questions, please reference the CDC’s and WHO’s resource guides.

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