Above: Passengers wearing protective facemasks amid concerns of the COVID-19 coronavirus outbreak wait to board their plane at Kuala Lumpur International Airport in Sepang on February 19, 2020. (Jewel Samad/AFP via Getty Images)
As the novel coronavirus wreaks havoc on societies and economies around the world, many are wondering if the return of summer might put a crimp in the virus’s spread across northern countries, including the United States.
The short answer is that a summertime lull in this coronavirus is possible – but it’s far from a sure thing, and any benefits might be limited.
As I discussed in a 2018 post, there’s a substantial body of research showing that influenza tends to peak in the cold season and wane in the summer across midlatitudes. The key meteorological factor isn’t the heat – it’s the humidity. At higher humidity, the virus appears to be less stable, and the small virus-bearing droplets sent into the air by a cough seem more likely to attract water vapor and fall out of the air before infecting someone else. There’s also evidence that the flu takes hold in the upper respiratory tract more readily during dry weather.
This moisture effect may not be large, but it’s robust and significant, according to Jeffrey Shaman (Columbia University), a leading researcher on the flu-weather connection.
Work carried out by Shaman and others indicates that absolute humidity – the total amount of water vapor in the air – is a particularly useful index. In a 2017 study, Alan Barreca (University of California, Los Angeles) and colleagues found that incorporating absolute humidity leads to a 3-5% improvement in forecast accuracy for models of flu transmission extending out to 4 weeks.
Climatological values of absolute humidity helped even more than the values drawn from weather forecasts, perhaps because of the low skill in multi-week weather forecasts.
What about the coronavirus?
The question at hand is whether such seasonal clues might apply to the brand-new coronavirus and the disease it causes (COVID-19). Coronaviruses are separate from influenza viruses.
According to Shaman, there are seven coronaviruses known to be capable of human-to-human transmission: SARS, MERS, the novel (new) coronavirus (SARS-CoV-2), and four endemic viruses.
“These last four viruses circulate every year and typically produce mild cold symptoms,” said Shaman in an email. Like the flu, these have a pronounced seasonal cycle: They peak in the winter (usually January or February) and are much less prevalent from mid-April through summer and early fall.
Will SARS-CoV2 exhibit a similar seasonality? “It may ultimately, but right now, given that it is a newly emerged virus to which most of the world is susceptible, I don’t think it will abate in April,” Shaman said. “Rather, it might ramp down in the U.S. in late May or June.”
This type of delayed seasonal response tends to occur when a new strain of type A flu spreads across the globe (pandemic influenza), according to Shaman.
“Due to high susceptibility, the pandemic virus continues to circulate through late May or June, is limited in summer, and ramps up again in September,” he said. “If SARS-CoV-2 follows suit, we could have a break from it in the summer, which would buy some time to further prepare. However, it would still be in circulation in the Southern Hemisphere…and in the tropics throughout the year, in all likelihood.”
Importantly, Shaman added, “All this assumes SARS-CoV-2 has a strong innate seasonality.” If not, he said, “it may just continue through summer unabated.”
In a Forbes essay “Will Spring Temperatures Stop Coronavirus?”, Marshall Shepherd (University of Georgia) reiterated the point that that flu viruses can thrive year round in the tropics and that researchers aren’t sure why.
Without a vaccine, what will next autumn bring?
It appears highly unlikely we’ll have an approved vaccine for SARS-CoV-2 before the end of 2020. This raises concern about any potential for a surge at northern latitudes during the coming autumn.
Epidemiologist Marc Lipsitch, who directs the Center for Communicable Disease Dynamics at Harvard University, got wide attention recently for his projection that many if not most people on Earth – perhaps 40 to 70% – could end up getting the novel coronavirus.
As the Atlantic writer James Hamblin put it, “Lipsitch is far from alone in his belief that this virus will continue to spread widely. The emerging consensus among epidemiologists is that the most likely outcome of this outbreak is a new seasonal disease – a fifth ‘endemic’ coronavirus. With the other four, people are not known to develop long-lasting immunity.
“If this one follows suit, and if the disease continues to be as severe as it is now, ‘cold and flu season’ could become ‘cold and flu and COVID-19 season’.”
Even if the novel coronavirus becomes endemic, there is huge value is slowing down the process. One of the main goals with the stringent control measures now taking place in many countries – including limits on public gatherings and the recommendations for frequent hand washing and social distance – is to put brakes on the immediate spread of SARS-CoV-2, thus buying time and reducing the strain on health care systems.
We can’t expect summer to take care of this job for us.
Lipsitch has an excellent summary of coronavirus and seasonality at his Harvard website. According to Lipsitch, “The short answer is that while we may expect modest declines in the contagiousness of SARS-CoV-2 in warmer, wetter weather and perhaps with the closing of schools in temperate regions of the Northern Hemisphere, it is not reasonable to expect these declines alone to slow transmission enough to make a big dent.”
Along the way, Lipsitch busts several myths, including the idea that SARS disappeared on its own in 2003 as the weather got warmer (it was actually the concurrent timing of strict control measures), and that the novel coronavirus will behave like the common cold (we just don’t know yet).
“Even seasonal infections can happen ‘out of season’ when they are new,” Lipsitch says. “New viruses have a temporary but important advantage – few or no individuals in the population are immune to them. Old viruses, which have been in the population for longer, operate on a thinner margin – most individuals are immune, and they have to make do with transmitting among the few who aren’t.
“In simple terms, viruses that have been around for a long time can make a living – spread through the population ¬– only when the conditions are the most favorable, in this case in winter,” Lipsitch says.
“The consequence is that new viruses – like pandemic influenza – can spread outside the normal season for their longer-established cousins.”
The seasons of the 1918 flu pandemic
There’s an ominous historical precedent for a pandemic influenza that appeared in the spring but struck with ferocity in the fall. An outbreak of flu started in March 1918, with more than 100 cases reported at a Kansas military base. Sporadic cases were reported in the United States and Europe later that spring and summer, according to a timeline from the Centers for Disease Control and Prevention.
The next and much more intense phase of this pandemic struck the United States in September and October 1918. In the month of October alone, the flu killed an estimated 195,000 Americans. That’s roughly one out of every 500 people in the entire nation, or comparable to 600,000-plus people today.
As for the novel coronavirus of 2020 and the question of its seasonal nature, Lipsitch has a pithy answer at the top of his website:
Seasonality of SARS-CoV-2: Will COVID-19 go away on its own in warmer weather?
(tl;dr) Probably not.
