The coronavirus dominated both summer, spring, and much of winter in 2020, seemingly impervious to seasonal change. However, it may still be too soon to abandon the idea that coronavirus, like the flu, can wax and wane with the seasons.
A study published Tuesday suggests it's possible and offers an explanation for why we haven't seen it happen yet.
In the United States, the coronavirus raged in the summer, a time when seasonal respiratory infections (like the flu) tend to fall. While that might seem like evidence that every season is Covid-19 season, this paper offers an explanation: Because the virus took hold in an unsuspecting world with no Covid-19 immunity, weather alone could not control it.
As the basic reproduction number of SARS-CoV-2 (the number of people infected by contact with one infected person) falls below 1 – a threshold at which the number of cases would decline – we may start to see patterns of seasonal illness emerge, the authors argue.
In other words, as the world becomes more immune to the virus, the virus may become seasonal, explains Hassan Zaraket, an assistant professor at The American University of Beirut and the paper's last author.
"Seasonal coronaviruses represent up to 30 percent of respiratory infections every year and hence, they pose substantial health and economic burden," he tells Inverse. "We expect that factors affecting seasonality will act similarly on COVID-19 but only when the transmission rate drops."
Arnold Monto is a professor of Epidemiology at the University of Michigan who has studied seasonal coronaviruses. He was not a part of this new study.
He says that it's "reasonable" to suggest that, earlier in the year, the pandemic was spreading too fast and too widely due to human behavior to really allow seasonal patterns to emerge.
"I think everything will depend on what happens if this becomes an endemic virus – which means it infects populations on a regular basis," he tells Inverse. "It may turn out to be more seasonal. The general answer is we're not sure."
What did this summer tell us – Initially, there were hopes that Covid-19 would abate as winter transitioned to summer. Some speculation came from President Trump, who said the virus might "miraculously" disappear with warm April weather.
Several pre-print (not peer-reviewed) papers from China suggested that increases in temperature would negatively affect the spread of the virus. However, as one of those papers points out, the weather alone can't reduce the spread of the virus to a controllable threshold.
"Public health intervention such as social distancing is crucial to block the transmission of COVID-19 even in summer," the authors of one March 2020 pre-print wrote. Months later, the US had a record wave of summer coronavirus infections as social distancing restrictions were lifted.
"Thus far, given the fact that we had a lot [of cases] over the summer, people have dismissed seasonality," Monto says. "We didn't know in February and we still don't know."
Monto's own recent, published in April, examined the seasonal trends of coronaviruses that cause the common cold (HCoVs, collectively) over eight years. Based on 993 infections, the team found evidence of clear seasonal trends.
The illnesses caused by those coronaviruses picked up steam in December, peaked in January or February, and began to decrease in March, the paper showed. Only 2.5 percent of cases occurred between June and September.
Though evidence on why these coronaviruses spread in the winter months isn't discussed in Zaraket or Monto's papers. However, evidence from the flu (which is also seasonal, and a contagious respiratory virus) demonstrates it spreads because people spent more time indoors, close to one another. Experiments have also regularly shown that the virus tends to survive better in colder, drier air.
Studies on the spread of SARS-CoV-2 have suggested that the virus spreads in drier air as well: A study on humidity and coronavirus cases in Australia in Transboundary and Emerging Diseases found that a one percent decrease in humidity was linked to about a 7.7 percent increase in the number of coronavirus cases reported.
(However, humidity likely plays a greater role in the spread of the virus indoors rather than outdoors, as Karen Kormuth, an assistant professor of biology at Bethany College told Inverse previously.)
Zaraket argues that we can look to other coronaviruses to inform what might happen as the pandemic transitions out of the current state – either through vaccination or acquired immunity over time – and infection slows.
Lia van der Hoek, a researcher at the Laboratory of Experimental Virology, also tells Inverse it's fair to look at common cold coronaviruses as models to predict how Covid-19 may behave.
"We see that all common cold coronaviruses have this seasonal preference," She explains. "It is way more likely that SARS-CoV-2 will act similarly in the post-pandemic period, than that it will not."
What would a seasonal Covid-19 really mean? – At this point, it's still too early to know whether SARS-CoV-2, the virus that causes Covid-19, may truly become seasonal, even if that's the pattern set forth by other coronaviruses.
"Predicting is different from observing what is really happening with SARS-CoV-2," van der Hoek says.
And we still don't know if a vaccine will simply lessen severity of the disease or help to stop infection. We also don't know how long immunity lasts, and whether reinfection (which has been reported in case studies) is truly a concern.
With these unanswered questions and no precedent for a coronavirus transitioning from a pandemic to a seasonal virus, we still aren't even sure what hallmarks would indicate that this is going to happen, says Monto.
At this point, Monto speculates that even if the coronavirus does become seasonal, it's not a guarantee the world would continue to retreat into lockdown each time the virus surges. The hope is, by that point, we have treatments that could prevent severe disease or a vaccine that can at least lessen its severity — even if it can't stop infection entirely.
"At that point, we should have other therapeutics and vaccines available like with flu," he says.
Hopefully, we're taking the steps towards controlling the disease. Even if we have to live with it for the foreseeable future.