How diseases rise and fall with the seasons—and what it could mean for coronavirus

How diseases rise and fall with the seasons—and what it could mean for coronavirus


–Music– 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.