The U.S. is now firmly in the middle of a second peak of the covid-19 pandemic, if still nestled inside its first wave. But given the rapid spread of the coronavirus during a time of the year when many similar diseases tend to lay low, it’s worth exploring: Why isn’t the summer stopping or at least mitigating the coronavirus, as many had hoped it would?
Many infectious diseases are seasonal, including other coronaviruses that cause the common cold. The most well-known example is flu season, which tends to start in the late fall and run through early spring in the U.S. Like the flu and lots of other seasonal viruses, covid-19 is primarily spread through our respiratory tract. So it wasn’t a far stretch to think that seasonality would have come into play for covid-19 this summer.
But there are complicated reasons why a specific illness is or isn’t seasonal. Many scientists early on in the pandemic were already skeptical about the summer being a magic bullet for covid-19 in the U.S., even as people like President Trump continued to insist that the pandemic would just “go away” due to the warmer weather.
The reasoning behind a theoretical summer slowdown for covid-19 is simple enough. The coronavirus has a fatty outer layer, or lipid membrane. High temperatures tend to degrade these outer layers, so the virus survives for less time in the environment than it does in the cold. The mugginess of high humidity can also impede how far droplets and aerosols from our mouths and noses travel before falling to the ground, while low humidity aids its survival and even seems to weaken our natural defenses against infection. And the Sun’s ultraviolet rays kill many viruses, acting as a natural disinfectant. But there are other important factors that influence a virus’s spread in a population, and clearly—given covid-19's recent surge across the U.S.—those factors are outweighing any summer effect that might exist.
The flu and other seasonal viruses are what scientists call endemic diseases, which means they’re constantly around and making some people sick. It also means a sizable segment of people will have some amount of immunity to them that might prevent infection or make reinfection milder. For some diseases, it’s thought their timing is influenced not just by weather conditions but by the level of immunity in a population (this may explain why some viruses tend to surge every two years).
Covid-19, unlike these other diseases, is brand new, so the baseline level of immunity is much lower than other seasonal respiratory viruses. There is evidence emerging that some people may have a pre-existing T cell immune response to the virus, borrowed from dealing with other common cold coronaviruses. But it’s not yet clear if and how having this response affects a person’s interaction with the virus, including their risk of catching and spreading it further. In any case, it’s still true that most of us haven’t encountered this specific coronavirus, and that likely greatly dampens any potential effects of seasonality.
“The lack of population immunity ends up being the key factor driving the spread of novel viruses such as SARS-CoV-2,” Rachel Baker, a postdoctoral research associate at Princeton University and the lead author of a recent study about the potential role of seasonality in covid-19, told Gizmodo via email. “This factor swamps any signal from the climate. When everyone is susceptible to contracting a new virus, it is able to spread really well, no matter the climate conditions.”
In a few years time, with building population immunity, Baker added, it’s possible that covid-19 could become a seasonal disease like the flu, assuming that no effective vaccine comes along by then.
Another consideration is a sort of paradoxical effect of summer. The theory goes that, even if the heat and strong sunlight of the summer do affect how easily the virus spreads (by making it harder for the virus to survive for long outside its host), scorching temperatures in places like Florida also drive people indoors more, where they’re then more likely to transmit and catch the virus. Poor ventilation and recycling of air indoors means that infectious droplets and aerosols stick around longer in greater quantities, increasing a risk of transmission.
Scientists are trying to pin down how weather affects the transmission of the coronavirus, which is no easy task. Recently, NASA announced that it created a database in collaboration with other space agencies to help researchers determine how seasonality could affect covid-19's spread, as well as other global impacts caused by the pandemic. As a NASA Earth Observatory press release describes it:
One way that [Johns Hopkins University scientist Benjamin] Zaitchik and other researchers hope to get some clarity is by making use of reanalysis datasets and models that synthesize disparate environmental data into a coherent whole. These systems generate consistent snapshots of Earth’s atmosphere and land and water surfaces over large areas and long time periods. This makes them especially useful for research on long-lasting global problems like climate change or the seasonality of an infectious disease.
Even if the U.S. summer isn’t stopping covid-19, that doesn’t necessarily mean it’s not slowing it down a little. But it’s likely that any summer effect hasn’t been enough to counter the collective weight of other things that enabled the virus to spread, like the reopening of bars, limited mask usage, and reduced distancing, especially in areas where community transmission was relatively high or increasing in the first place.
“Whenever you have individuals grouping together indoors, without practicing social distancing measures, we should expect this virus to spread really well,” Baker said. And while there may be truth to the idea that an especially hot summer is driving more people indoors, Baker places more blame on states lifting their stricter control measures too early.
Unfortunately, even if the summer is having a slight or no effect on transmission right now, the fall and winter could still have a potent opposite effect. Based on what we know about other coronaviruses and other pandemics caused by respiratory viruses, many scientists remain worried that the cold and dry weather of fall and winter will provide a boost to the pandemic. Indeed, the flu pandemics of 2009 and 1918 both had at least two waves in the U.S., with both second waves picking up steam in the fall. In those pandemics, the second waves turned out to be deadlier than the first, though that’s not a guarantee the same will happen with covid-19.
What is certain is that the U.S. is still losing ground to this pandemic, while many other countries have far fewer daily cases and have started to regain some semblance of normal life. That really doesn’t bode well for what might come next. According to one model that’s been especially accurate so far, the U.S. is expected to reach 200,000 official covid-19 deaths by October 1, just as fall begins.
“I think it’s likely that transmission will increase in the winter, however, we should expect large outbreaks of covid-19 whenever we step back from social distancing measures, irrespective of the climate,” Baker said. “I think the thing to watch out for in winter months is possible joint outbreaks of covid-19 and other seasonal viruses such as influenza. We are still unsure how the two viruses might interact, and, either way, this could put additional pressure on the healthcare system.”