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How exactly do we become “under the weather”?
The question has fascinated the field of medicine for millennia.
Observers of disease, from Hippocrates to Salk, have noticed that illnesses ebb and
flow with the seasons.
Influenza, for instance, goes away in the summer and comes back in late fall or winter.
Now that a new disease, COVID-19, is spreading around the globe, scientists, governments,
and the public are wondering: Could the arrival of summer in the Northern hemisphere cause
this virus to lose steam as well?
Dozens of other diseases are known to be seasonal.
Rotavirus spreads well in the Southwest United States in mid-winter but prefers spring in
the Northeast.
Syphilis seems to favor the winter in China.
So does hepatitis C in India.
Before the vaccine was invented, polio was a summer disease in the United States.
Swimming pools were often closed to halt its spread.
But only a tiny club of researchers specializes in seasonality of diseases, and there’s
still a lot they don’t understand.
One hypothesis is that viruses are better able to survive in some seasons.
When an influenza patient sneezes, the flu virus is dispersed in thousands of tiny droplets.
These droplets evaporate at different rates due to seasonal changes in humidity and temperature,
researchers suspect.
That may destroy the fragile outer membrane, or envelope, that surrounds each flu virus.
Another factor could be that people spend less time indoors in the summer.
Or changes in people’s diets.
Micaela Martinez, an infectious disease ecologist at Columbia University, is exploring another possibility:
What if our immune system changes with the seasons?
Scientists know this happens in animals.
This little bird, the stonechat, ramps up its immune system to kill bacteria in the
summer before migrating, which requires a lot of extra energy.
One study in humans found that influenza vaccines given in the morning triggered a stronger
response than one administered in the afternoon, suggesting immunity
has a daily, or circadian, hythm.
It’s influenced by melatonin, a hormone
which surges at night and tapers at sunrise
Martinez has set up camp at University of Surrey in England, where a unique lab allows
her to monitor people’s body changes under rigidly controlled lighting conditions.
In December, 12 men and women checked into the lab.
Their noses were swabbed to screen for 16 different respiratory viruses.
They each had their own temperature-regulated rooms,
where they remained for 24 hours under dim lighting.
Researchers repeatedly took blood samples during that day.
They want to find how the different light in winter, spring, summer and fall affects
the many populations of immune cells in the body--and how their changing numbers and locations
in the body affect our ability to ward off different diseases.
So what does the science of seasonality tell us about COVID-19?
Unfortunately, not much.
The new disease only surfaced in December.
Scientists say it’s just too early to say if it is seasonal.
Other human coronaviruses which cause the common cold do peak in winter.
But those have long been circulating in the human population.
Because COVID-19 is new, nobody has immunity to it.
So even if conditions are less hospitable to it in summer
the virus may still keep spreading—
hopefully at a lower level.
With much still unknown about seasonal effects on infectious diseases
Learning the facts will require the passage of time.