As winter brings shorter days and lower temperatures to the Northern Hemisphere, there is a chance we could see more COVID-19 cases.
But experts say it is still too early to know exactly how seasons will affect the virus. They emphasize that human behaviors are still the most important driver of the pandemic.
“The most important factor at the moment is ... the control measures that we have in place. Things such as social distancing and mask-wearing — those are really key to lowering transmission of disease at this point,” said Rachel Baker, infectious disease researcher at Princeton University.
COVID and climate
Many diseases, such as the flu, are seasonal, with cases spiking when the weather is cool and dry.
“It’s well known that many respiratory viruses have seasonality,” said Akiko Iwasaki, professor of immunology at Yale University. “And so, I wouldn't be surprised if there's actually elevated transmission during the winter months from COVID.”
There are three main reasons why scientists think the coronavirus could be affected by climate.
“The virus doesn’t like certain seasons, or our bodies don’t like certain seasons. Or it’s just that we’re putting more of our bodies together in closed spaces,” said Ben Zaitchik, associate professor of Earth and planetary sciences at Johns Hopkins University.
SARS-CoV-2, the official name of the virus that causes COVID-19, is spread by respiratory droplets produced when people breathe, talk, sneeze or cough. The virus survives better in cold, dry conditions typical of temperate winters. Low humidity also promotes evaporation of virus droplets into tiny aerosol particles that linger in the air, increasing the risk of airborne transmission in winter.
Cold weather may further increase disease spread by driving us indoors.
“The longer that household contacts are together, the more likely they are to transmit [the virus],” John Lynch, professor of infectious diseases at the University of Washington, said at a press conference. “When we think about the places where we’re seeing transmissions occur, it’s mostly homes. It’s mostly constrained workplaces where people don’t have the ability to separate from each other.”
Winter weather can also hinder the body’s ability to fend off viral infections. A lack of sunshine may deplete vitamin D levels and weaken the immune system, and one study found a link between low vitamin D levels and COVID-19 cases.
Cold, dry winter air also damages the cells in our airways that clear away virus particles. If your body can’t get rid of these virus particles, it might take fewer virus particles to make you sick, or the disease might be more severe, Iwasaki said.
Iwasaki recommends that people use humidifiers to moisten the air of homes and offices. She also said that masks can help.
In addition to reducing spread of the virus, “another thing [a mask] does is it warms the nose and moistens the respiratory tract. So, I think masks are a great idea for multiple reasons, just even to boost this moisture inside the respiratory tract to better fight off the infection,” Iwasaki said.
How will winter affect COVID?
There’s good reason to believe that COVID-19 is sensitive to the seasons, and some studies have linked cold, dry conditions with outbreaks of the disease. But researchers say that weather takes a back seat at the early stages of a pandemic because everyone is susceptible to the virus.
“When you have a population with no immunity to the virus, it spreads really well, no matter the climate conditions. So it just kind of takes off,” said Princeton’s Baker.
In a new study, Baker predicts that human behaviors such as mask-wearing and social distancing will be the most important factors in slowing disease spread during winter in New York and other cities, although she notes that the study has not yet undergone a formal review process.
But in areas where disease cases are slightly declining, “it's possible that climate could give you enough of a boost of transmission to cause a large outbreak,” Baker said. “So, we are a bit worried that when winter comes, if you're in a place that gets really cold, dry winters, it might be enough to push transmission [up], and then you'd start to see a growth in cases.”
COVID-19: A future seasonal disease?
As more people develop immunity to the virus after overcoming an infection or through vaccination, researchers say that COVID-19 could become a seasonal disease, with numbers of cases oscillating between the seasons.
“As more of the population has had the virus, more of the population develops immunity to the virus,” Baker said. “As that immunity increases, then you'll start to see more effects of climate.”
However, the future of the disease will be shaped by what this immunity looks like — a question that has not yet been answered. Researchers predict that the timing and number of COVID-19 cases will hinge on the effectiveness of potential vaccines and how long immunity lasts.
Disease predictions were very different depending on whether “SARS-CoV-2 is a type of virus where you get it once and you're done, you can never transmit it again, you can never get infected again,” compared with “if it's a type of virus where you might have some amount of protective immunity, but you could potentially be reinfected again,” said Caroline Wagner, assistant professor of bioengineering at McGill University.
Slowing the spread
Experts emphasize that the course of the pandemic is still largely in our hands.
“What we know works is social distancing, wearing masks or face coverings, and practicing good hand hygiene. If we stick with those things, we’re going to have really good success with interrupting transmission,” said the University of Washington’s Lynch.
Jeanne Marrazzo, professor of infectious diseases at the University of Alabama at Birmingham, agrees that we have the tools to deal with the potential of more cases in winter.
“We know what to do. The question is, do we have the social, political and economic will? I think we have the medical will. We have the public health will. It’s just a question of, ‘Can we mobilize the community to continue to exercise the kinds of caution we need?’ ” said Marrazzo at a press conference.
“As we face the coming months, I really, really hope we can pull together to recognize that we can change the trajectory if we work together,” she said.