• Because flus and colds are less prevalent during the summer, many people — including President Trump — hoped that warmer weather would stymie the coronavirus' spread.
  • The US's recent surge in infections, notably in southern states with high temperatures, has thrown cold water on that idea.
  • The coronavirus isn't seasonal yet because very few people are immune to it, so it spreads easily.
  • It might take two to three years for the coronavirus to enter into a seasonal pattern.
  • Visit Business Insider's homepage for more stories.

President Donald Trump offered an optimistic prediction five months ago: The coronavirus would disappear with the advent of warmer weather.

"A lot of people think that goes away in April, with the heat that comes in," Trump told US governors on February 10, adding, "the heat, generally speaking, kills this kind of virus."

The logic was that because flus and common colds retreat during the summer, the coronavirus might, too. But summer is here, and the virus' spread has only accelerated in the US. Florida reported the highest single-day coronavirus surge in a state ever recorded —  15,300 cases — on Sunday. The state's high temperature that day: 95 degrees Fahrenheit.

Arizona, California, and Texas, too, have reported unprecedented spikes in infections, while their average high temperatures hover between 80 and 106 degrees. 

There's a reason the coronavirus is spreading rapidly no matter the weather, according to Rachel Baker, a researcher at Princeton's Environmental Institute: "We're at the start of pandemic when a new virus is emerging into a population that hasn't had it before," she told Business Insider. "So a lack of population immunity becomes a key driver of spread, and climate doesn't really matter very much at first." 

Seasonality only comes into play when a large chunk of the population is immune to a virus

Even during the winter, most researchers questioned Trump's assertion that the spring and summer would bring relief.

"One would hope that the gradual spring will help this virus recede. We can't be sure of that," William Schaffner, an infectious-disease specialist at Vanderbilt University, told CNN in February. 

Baker recently simulated the effects of climate on the spread of the coronavirus in cities around the world, based on how coronaviruses that cause common colds spread in different climates and seasons.

Her team's results, published in May in the journal Science, showed that warm weather only curbs a virus' spread after a large chunk of the population becomes immune or resistant to infection.

"Think of it like a sliding scale: As population immunity builds up, you see a bigger climate signature in controlling spread," Baker said.

For now, our lack of immunity to the new virus gives it  "a temporary but important advantage," according to epidemiologist Marc Lipsitch.

"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," he wrote in a March article for Harvard University, referring specifically to the flu.

Never-before-seen viruses, by contrast, "spread outside the normal season for their longer-established cousins," Lipsitch added.

It's possible, however,  the coronavirus will "settle into that classic seasonal pattern with a peak in winter months" after about two to three years — once a vaccine is developed and distributed, Baker said. 

Temperature and humidity can affect the coronavirus' spread

Respiratory viruses like the flu are seasonal because cooler temperatures help harden a protective gel-like coating that surrounds the virus while it's in the air. A stronger shell ensures it can survive long enough to travel from one person to the next.

But that sheath dries out faster in warmer temperatures.

Research published last month revealed that warmer temperatures and lower humidity can truncate how long the new coronavirus survives on surfaces, which may in turn hinder spread. That could explain why New York City had a higher growth rate of new infections compared to Singapore in March, the scientists said, though other factors like testing and contact tracing likely played a role as well. 

Similarly, an April study found a link between the virus' lifespan and the surrounding temperature. At 39 degrees Fahrenheit, the coronavirus lasted up to two weeks in a test tube. When the temperature was turned up to 99 degrees Fahrenheit, that lifespan dropped to one day.

Additional, though preliminary, research suggests that for each degree temperature rises and each percentage point humidity increases, the average number of people a coronavirus patient infects goes down.

Still, the impacts of temperature and climate on the virus' spread are ultimately negligible in the absence of efforts to flatten the curve. 

"To the extent that there are any seasonal effects, they're washed over by these much stronger, broader effects of whether we're controlling it — like how good a job we're doing at masking," Tom Frieden, former director of the Centers for Disease Control and Prevention, told The Wall Street Journal on Monday.

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