As temperatures warm up, public officials and concerned citizens alike are hoping for the same decrease in cases of the disease caused by the novel coronavirus (COVID-19) that we’ve come to expect from the seasonal flu.
"A lot of people think that goes away in April, with the heat, as the heat comes in," President Trump suggested in February, though he didn’t offer a medical explanation of why that could be.
There are several factors that could lead to a decline in COVID-19 in the coming months. But with so much still unknown about this new virus, experts say it’s difficult to predict whether this coronavirus will behave like the common cold or influenza.
The Centers for Disease Control (CDC) has said the effects of outdoor temperatures on coronavirus are uncertain.
“At this time, it is not known whether the spread of COVID-19 will decrease when weather becomes warmer,” the CDC website says. “There is much more to learn about the transmissibility, severity, and other features associated with COVID-19 and investigations are ongoing.”
“There is some hope that it will decline, but I don't think we can count on that,” Eli Perencevich, a professor of epidemiology and internal medicine at the University of Iowa Carver College of Medicine, told Yahoo News. “Right now we don't know. This is a brand new virus, and clearly this is much larger than any previous outbreak like SARS or MERS. So we just don't know.”
Nelson Michael, director of the Center for Infectious Diseases Research at Walter Reed Army Institute of Research, also expressed cautious optimism during a briefing on COVID-19 on March 5. "This is a respiratory virus and they always give us trouble during cold weather," he said, predicting that the coronavirus may react like the flu and cause "less trouble as the weather warms up."
Perencevich points to several reasons why the flu and common cold have seasonal patterns that COVID-19 might follow. Increases in temperature and humidity, Perencevich says, are less favorable conditions for flu transmission — an observation shared by other epidemiologists including Perencevich’s colleague, Marc Lipsitch, director at the Center for Communicable Disease Dynamics at Harvard.
“For influenza, it has been elegantly shown in the lab that absolute humidity — the quantity of water vapor in the air — strongly affects flu transmission, with drier conditions being more favorable. Subsequently it has been shown that epidemiological patterns are consistent with this lab data in the US and in Vietnam, among other study sites. Notably the Vietnam study looked at influenza-like illness, without distinguishing influenza from other types of pathogens. This hints that similar mechanisms may be at work for other respiratory viruses, but to my knowledge are no specific studies of the role of humidity for coronaviruses or other respiratory viruses besides flu,” Lipsitch wrote in a post for the Harvard T.H. Chan School of Public Health.
Warmer temperatures also result in people spending more time outdoors, where increased ventilation and personal space can reduce the risk of disease spreading. Cramped classrooms that are closed during summer vacation, for example, could prevent children from catching and sharing COVID-19. But with so few cases of children diagnosed with COVID-19, it is unclear whether children aren’t easily infected, or if they simply get milder cases and can still infect others.
Lipsitch points out that seasonal viruses that have been in the population for a long time behave differently from viruses that are newly introduced into the population. And there’s already reason to believe that warmer temperatures may not be much help. Countries like Australia and Argentina, for example, are located in the southern hemisphere where it’s already summer, and influenza season doesn’t begin until May. Yet these countries have already reported locally transmitted cases of COVID-19, with 140 cases in Australia alone as of March 13.
“We have no reason to believe that this virus would behave differently in different temperatures, which is why we want aggressive action in all countries to make sure that we prevent onward transmission, and that it’s taken seriously in every country,” said infectious disease epidemiologist Maria van Kerkhove during a World Health Organization press conference on March 5.
Even if COVID-19 does wane in the summer, seasonal influenza and past pandemics such as the 1918 Spanish flu and the 2009 pandemic flu demonstrate that COVID-19 could return with vengeance when temperatures drop again.
“We have to be ready for next winter when it may come back again,” Michael cautioned during the March 5 press conference, as he stressed the need for a vaccine.
Finally, Perencevich noted that because this is a new virus, we ultimately may not see a declining number of cases of COVID-19 until the virus has made its way through the population and enough people have become immune.
“There's a concept called ‘herd immunity,’” Perencevich explained. “Once the population — whether that's a country or the entire planet — crosses a threshold of people who have had the infection and recovered, they become immune. Then the infection can decline and disappear.”
“Numbers that people have mentioned — perhaps forty to seventy percent of certain countries or the world's population will become infected. And that's largely a calculation of the herd immunity threshold. And so once you reach that, then we expect infections to really decline. And the reason for that is just a math probability issue. If someone's infected and they can potentially spread it to two or three people, if those two or three people are immune, then they won't really spread it efficiently.”
But Perencevich noted that there is no timeline for how long it could take for enough people to build up immunity and stem the spread of the virus.
“It depends on the infectivity of the virus, the size of the population. We just don't know.”
“We have to continue doing everything we can to do social distancing, avoiding large crowds and trying to not get sick,” Perencevich added.
“That's probably the best way to have it decline soon: to wash our hands and keep a distance.”
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