Quotes from Experts

Changing seasons and COVID-19

SciLine reaches out to our network of scientific experts and poses commonly asked questions about newsworthy topics. Reporters can use these responses in news stories, with attribution to the expert.

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November 16, 2020


How might changes in temperature or in social behavior affect COVID-19 transmission during the winter?


“The primary factor set to influence COVID-19 transmission in the winter months is the strength of the control measures we have in place. In places where the pandemic is barely under control, wintertime climate conditions may lead to a small increase transmission, which could be enough to cause a surge in cases. However, locations that already taking substantial steps to limit the spread of the virus, including mask wearing and social distancing where possible, should be able to limit the risk of a large secondary outbreak.” (Posted November 16, 2020)

Rachel E. Baker, PhD
Associate Research Scholar, Princeton University

“Early in a pandemic, behavioral seasonality has a much bigger effect than environmental seasonality. The biggest effect of temperature will be on how people move. As people move their socializing indoors—especially if indoor dining or bars stay open—we expect to see massive spikes in cases. If people go home for the holidays, they’ll take COVID with them. But it ultimately comes down to our choices. Now, in a winter two to three years from now, there might be enough population immunity that case totals better track shifts in the weather. Immunity helps diseases establish transmission cycles; if COVID becomes truly endemic, that’s when we start worrying about environmental seasonality.” (Posted November 16, 2020)

Colin J. Carlson, PhD
Assistant Research Professor, Center for Global Health Science & Security, Georgetown University

“Our research suggests that weather variation is correlated with COVID-19 risk around the world.

A model we created based on weather variation during the early days of global transmission predicted that COVID-19 would decrease in the summer in the northern hemisphere, return by autumn, and peak this winter. Unfortunately, these predictions appear to be coming true in many parts of the world.

“Perhaps this is not surprising – many human viruses also cycle seasonally and are associated with particular weather patterns. However, the observed correlations between weather and COVID-19 suggest that the virus might be more susceptible to weather and seasonality than other viruses.

“The most important weather variable for predicting COVID-19 growth in our work was ultraviolet light – research suggests that sunny days are linked to lower COVID-19 growth rates. We might not just want to bask in the warmth of the sun these days, but also bask in its protections from COVID-19.

“Because daylength varies seasonally, we predicted a distinct seasonal pattern to COVID-19, such that the southern hemisphere was expected to peak during their winter in July and August and the northern hemisphere is expected to hit its peak in December and January.

“This link between COVID-19 and ultraviolet light could be due to direct inactivation of the virus, promotion of human viral resistance through vitamin D production, or changes in human behavior as we congregate indoors during cool or rainy weather.

“Recently, we also discovered that certain subgroups, called clades, of the SARS-CoV-2 virus family are more weather-dependent than others. In particular, clade 20 of the SARS-CoV-2 virus seems to spread faster when coupled with certain weather conditions such as low humidity. Our projections suggest that, without intervention, COVID-19 has the potential to grow more quickly this winter in regions dominated by clade 20, including most of South and North America.” (Posted November 16, 2020)*

Mark C. Urban, PhD
Director of the Center of Biological Risk, Professor of Ecology & Evolutionary Biology, University of Connecticut

* Links included in quotes were supplied by the expert.


Does the body’s reaction to infections change during cold weather?


“Yes, the immune system can become a bit worse in the winter, increasing susceptibility to the virus and potentially leading to worse outcomes. But that information can be a bit extraneous sometimes; it’s hard to overstate the risk posed by COVID regardless of season or weather. Even in the nicest summer weather, this isn’t a virus you want to get.” (Posted November 16, 2020)

Colin J. Carlson, PhD
Assistant Research Professor, Center for Global Health Science & Security, Georgetown University

What should individuals do to reduce their risk of catching and spreading COVID-19 this winter?


“Of course, masks should be worn in all public environments such as in supermarkets, shops and crowded streets. I think individuals should pay attention to how their behavior is changing – are they tempted to dine-in at a restaurant when before they would have eaten outside? In general, individuals should try to avoid any indoor environment, including buses, planes, bars and restaurants, where they will be in prolonged contact with strangers. The big question for many will be whether to meet family members during the holiday period. To completely minimize risk, of course, individuals should not meet with family, especially those in vulnerable groups. However, if individuals do plan to meet with family, I’d recommend minimizing the number of households coming together – meet with one other household and make sure that neither household has met with anyone else in the previous 14 days. This may be particularly hard, or impossible, for families with children in school or daycare. Individuals should also pay attention to local guidelines on social distancing which will likely be updated in the coming months.” (Posted November 16, 2020)

Rachel E. Baker, PhD
Associate Research Scholar, Princeton University

“By now, you probably know the drill: limit your contacts when you can, get tested if you’re at risk, wash your hands, wear a mask, and be considerate of others. We’re also particularly worried that flu and COVID could create a ‘syndemic,’ where the two supercharge each other and lead to more cases and worse symptoms. So if you can, get a flu shot. But the best thing we can do is act as a collective to protect each other, and that means that governors and mayors need to move rapidly to intervene before the pandemic becomes even more uncontrolled.” (Posted November 16, 2020)

Colin J. Carlson, PhD
Assistant Research Professor, Center for Global Health Science & Security, Georgetown University

“We need to prepare now for a challenging winter in the northern hemisphere. Nature was on our side this summer, but it is not this winter. We will need to employ all the tools at our disposal, including social and pharmaceutical interventions, to get through winter and back to the sunny side of this pandemic.

“However, we need to be clear that high uncertainty remains in our forecasts of COVID-19 transmission, and this uncertainty should be incorporated into any decisions. In particular, the weather operates only in the background of social interventions. Maximum predicted rates of weather-fueled COVID-19 transmission need not be realized if we can find the right mix between disease potential and social restrictions.” (Posted November 16, 2020)

Mark C. Urban, PhD
Director of the Center of Biological Risk, Professor of Ecology & Evolutionary Biology, University of Connecticut

April 9, 2020


Why do some viral diseases show seasonality?


“The effects of climate on transmission of respiratory viruses are nonlinear, interactive, and combine multiple mechanisms.

”Together, these effects create very different patterns of seasonality in temperate regions and tropical regions, especially for influenza. In temperate regions, influenza peaks in the cold-dry season, while in tropical regions, it peaks in the warm-wet season, or has two peaks, one corresponding to each season.

”The relationships between climate and transmission for two very different respiratory viruses—influenza and respiratory syncytial virus—are very similar, suggesting that we might see similar patterns for SARS-Cov-2 in the coming years, after the virus becomes established and more people have immunity.

”However, right now during the emerging epidemic phase, human factors like travel and contact rates (due to shelter-in-place orders) are most important for determining the transmission rate. When almost the entire population is susceptible to the virus, transmission rates are probably too high for climate to have a strong effect.

”Overall, absolute humidity is the most predictive variable for transmission of respiratory viruses, but absolute humidity itself depends on temperature.

”Laboratory studies show a steep exponential decline in transmission with increasing absolute humidity (due to lower virus survival and possibly also droplets not staying afloat in the air as long), while statistical models fit to observed cases show a u-shaped response where transmission increases again at very high absolute humidities. This observed increase is linked to higher precipitation, but it’s not clear if the mechanism is a direct effect on virus survival in the environment or on human behavior (increased contact rates from congregating inside).” (Posted April 9, 2020)

Marta Shocket, PhD
Postdoctoral Fellow in the Department of Ecology and Evolutionary Biology at University of California Los Angeles

“Seasonality is most marked in temperate climates and abates as you move toward the tropics. In my view, we don’t really understand the determinants of seasonality, but they are not dependent upon seasonal differences in social behavior.” (Posted April 9, 2020)

Neal Nathanson, MD
Professor Emeritus of Microbiology, University of Pennsylvania

How do scientists determine the sensitivity of a virus to changing seasons?


“A combination of 1) laboratory experiments on the virus itself where we control the climate, 2) statistical models of observed cases that use natural variation in climate, and 3) mathematical models that take the relationships measured between climate and virus performance in the laboratory and simulate what we expect to happen in human populations.” (Posted April 9, 2020)

Marta Shocket, PhD
Postdoctoral Fellow in the Department of Ecology and Evolutionary Biology at University of California Los Angeles

“There are two ways; experimentally in the laboratory and observationally by monitoring changes in occurrence of new infections in the human population. Each approach has strengths. As we’ve seen described in some recent publications a laboratory will take a known amount of virus and place it on different surfaces or in the air at different combinations of temperatures and humidity and then sample the surfaces or air at various times to see how much viable virus remains. Many different viruses seem to remain viable for longer times at lower temperatures and drier air conditions. This is a very direct and important type of study but it has a major limitation. The experimental approach does not answer how much virus does it take (i.e,. The dose) to infect the ‘typical’ human. Even if the lab study says that it can detect virus at 3 hours or 3 days after the experiment started that doesn’t mean that there is enough virus remaining to infect a person. That is where the observational (epidemiologic) studies are critical. You get that information using standardized, well-designed population surveys to get an idea of how fast (and where) the virus is spreading in the population and whether transmission rates are increasing or decreasing.” (Posted April 9, 2020)

Gregory E. Glass, PhD
Professor in the Department of Geography & Emerging Pathogens Institute; University of Florida

“Each virus has its own distinctive peak and trough. If you look at enough different viruses, you will find some that peak at every time of year; same for trough. Also, viruses with similar structures can show different peaks, so you can’t sort viruses by structure and find a consistent pattern of peaks and troughs. My working hypothesis is that each virus has its own ability to survive under specific conditions of temperature and humidity and that this is an important determinant of seasonality.” (Posted April 9, 2020)

Neal Nathanson, MD
Professor Emeritus of Microbiology, University of Pennsylvania

Is SARS-CoV-2 sensitive to seasonal factors like temperature or humidity?


“I haven’t seen any data specifically for SARS-CoV-2—virologists are setting up some experiments right now—but influenza and respiratory syncytial virus are two different respiratory viruses that are both very sensitive to climate, especially absolute humidity—that itself depends on temperature—so we can guess that SARS-CoV-2 will also be fairly sensitive to climate, but only after it becomes more established.

”Another type of test, which will prove critical to identifying possible “exit strategies” for countries in various levels of lockdown, does not look directly for the virus, but rather looks for specific proteins that a person’s immune response produces after being infected. These proteins are called antibodies, and circulate in the blood, allowing for the potential development of easy-to-use, fingerprick-based tests that could even be done by people at home. However, we know from previous experience with other coronaviruses, as well as preliminary data from countries in East Asia, that antibodies are only usually produced in sufficient levels to be detected a week or more after the onset of symptoms. As such, antibody tests are not effective diagnostic tools to look for active cases of COVID-19, but will be extremely useful in identifying those who have previously been infected, and have since recovered. A study from China has suggested that 100% of the patients infected with SARS-CoV-2 had detectable levels of virus-specific antibodies in their blood two weeks after symptom onset.

”Widespread deployment of these types of antibody tests (often referred to as serological tests) will allow public health authorities to better understand the extent of viral transmission in the community. Knowing how many people have been exposed and have recovered will greatly improve predictive modeling efforts and help determine potential “exit strategies” from current lockdown measures.” (Posted April 9, 2020)

Marta Shocket, PhD
Postdoctoral Fellow in the Department of Ecology and Evolutionary Biology at University of California Los Angeles

If so, will new cases of COVID-19 be less common during summer months?


“Possibly. Quantitative modeling is still in progress, but the consensus among scientists is that climate is unlikely to substantially suppress COVID-19 on its own during the summer of 2020 because we will still have a population that is almost completely susceptible to the virus, unlike for influenza, respiratory syncytial virus, and colds, where much of the population has been previously exposed or vaccinated and has some pre-existing immunity. Accordingly, baseline transmission rates right now are much higher for COVID-19 than for those other diseases, so a seasonal reduction in transmission rate won’t have as big of an effect on the overall epidemic dynamics. We’re much more likely to see strong seasonality of COVID-19 due to climate in the coming years (summer of 2021 and beyond), once the virus is established in the population, a substantial portion of people have been exposed to it and have immunity to it—or have been vaccinated against it—and the baseline transmission rate is lower. Of course, this only applies to temperate areas. Tropical areas are likely to have increased transmission of COVID-19 during the warm and wet season, just like many of them do for influenza.” (Posted April 9, 2020)

Marta Shocket, PhD
Postdoctoral Fellow in the Department of Ecology and Evolutionary Biology at University of California Los Angeles

“If the virus viability declines at higher temperatures and humidity then you might expect a decline in cases but only if a substantial part of transmission is by indirect contact with contaminated surfaces. If most of the transmission is directly by aerosol from infectious person to susceptible person then cases might decline little if any. The seasonal analogy with other respiratory viruses was a good first place to start when we had no information. Certainly, countries where the epidemic occurred early are going to show a seasonal pattern but whether that is because of the weather conditions or because of the control measures implemented have taken effect we won’t know. In the southern U.S. We are into an early ‘summer’ and it doesn’t seem to have made the epidemics recede.” (Posted April 9, 2020)

Gregory E. Glass, PhD
Professor in the Department of Geography & Emerging Pathogens Institute; University of Florida

Does the COVID-19 outbreak in warmer parts of the world give clues about summertime transmission in North America?


“I don’t think so. Warmer parts of the world show very different patterns for climate-driven seasonality of influenza and respiratory syncytial virus transmission than temperate North America does. In tropical regions, the climate response is primarily driven by precipitation, and higher absolutely humidity increases rates of transmission instead of decreasing rates of transmission. Since those seasonal patterns for transmission of respiratory viruses in the tropics don’t apply very well to summertime in North America, I wouldn’t expect tropical transmission of COVID-19 to apply to North American summertime patterns either. It’s more likely that we’ll see a temperate-tropical divide in seasonality patterns for COVID-19 that is similar to the one we see for influenza and respiratory syncytial virus.” (Posted April 9, 2020)

Marta Shocket, PhD
Postdoctoral Fellow in the Department of Ecology and Evolutionary Biology at University of California Los Angeles

“If outbreaks continue and expand in the southern U.S. and parts of central and south America then I think it should make us pause considering any strategy that requires a major decline in cases due to weather. If cases do decline with summer conditions that is great and it may assist the medical community’s fight. But remember that even with seasonal influenza, it doesn’t go away entirely it just circulates at lower levels in some seasons than in others and I suspect covid-19 is here to stay.” (Posted April 9, 2020)

Gregory E. Glass, PhD
Professor in the Department of Geography & Emerging Pathogens Institute; University of Florida

Could climate change make pandemics like COVID-19 more likely?


“Climate change is having an unprecedented impact on global ecosystems. By making animals sicker and pushing them into new habitats, we expect that climate change could have downstream effects on the evolution of viruses at a global scale. That might help some viruses reach humans for the first time, and—in the worst cases—could lead to new kinds of spillover. For example, the most constant pandemic threat is still influenza, and the impacts of climate change on wild bird migration and virus circulation are still poorly understood, but could be major. But at the end of the day, increasing spillover doesn’t necessarily mean increasing pandemic risk: the difference between the two comes down to our choices, and our ability to detect outbreaks early, leverage strong health systems to respond, and prevent global spread.” (Posted November 16, 2020)

Colin J. Carlson, PhD
Assistant Research Professor, Center for Global Health Science & Security, Georgetown University

“Climate has big effects on how diseases spread once they emerge, but I don’t think climate and climate change are primary factors in new disease emergence itself. However, climate change is just one part of what ecologists call “global change.” This bigger term also includes things like land use change—deforestation and the expansion of agricultural and human settlements into new habitats—which we think increases contact between humans and wild animals and increases the chance of a rare event like a pathogen crossing into humans. Once a new pathogen gets into humans, our increased connectivity from a globalized economy with lots of international travel make it more likely that this pathogen will spread globally.” (Posted April 9, 2020)

Marta Shocket, PhD
Postdoctoral Fellow in the Department of Ecology and Evolutionary Biology at University of California Los Angeles

“I don’t see a scenario where climate change conditions that have been postulated will directly drive an increased likelihood of outbreaks. The spillover and emergence of viral outbreaks from wildlife are usually tied to increased human population size, the access to more natural resources, and increased movement of people regionally and globally. Those factors are also considered tied to climate change, so you could imagine that controlling the factors that drive climate change might also reduce the likelihood new pandemics will emerge, but it isn’t a direct link.” (Posted April 9, 2020)

Gregory E. Glass, PhD
Professor in the Department of Geography & Emerging Pathogens Institute; University of Florida

Creative Commons LicenseThe text and video on this page are licensed as Creative Commons CC BY-SA 4.0. Journalists are free to use any text or video on this page with or without attribution to SciLine.

Rachel E. Baker, PhD, Associate Research Scholar, Princeton University

I have no conflicts of interest.

Colin J. Carlson, PhD, Assistant Research Professor, Center for Global Health Science & Security, Georgetown University

No COI to declare

Gregory E. Glass, PhD, Professor in the Department of Geography & Emerging Pathogens Institute; University of Florida, said:

I was a professor at the Johns Hopkins Bloomberg School of Public Health in the Department of Molecular Microbiology & Immunology and the Department of Epidemiology for 30 years before being hired at University of Florida

I was one of the authors of the 4th U.S. National Climate Assessment working on the Human Health Chapter.

I am on the external advisory board for the Center for Health & The Global Environment, University of Washington

I hold several grants with the U.S. CDC and several contracts for health studies internationally.

Neal Nathanson, MD, Professor Emeritus of Microbiology, University of Pennsylvania, said:

None

Marta Shocket, PhD, Postdoctoral Fellow in the Department of Ecology and Evolutionary Biology at University of California Los Angeles, said:

None

Mark C. Urban, PhD, Director of the Center of Biological Risk, Professor of Ecology & Evolutionary Biology, University of Connecticut

No conflicts of interest noted.