“Why Do Dozens of Diseases Wax and Wane With the Seasons—And Will COVID-19?”, 2020-03-13 (; similar):
…a phenomenon recognized 2500 years ago by Hippocrates and Thucydides: Many infectious diseases are more common during specific seasons. “It’s a very old question, but it’s not very well studied”, Martinez says. It’s also a question that has suddenly become more pressing because of the emergence of COVID-19. With SARS-CoV-2, the virus that causes the disease, now infecting more than 135,000 around the globe, some hope it might mimic influenza and abate as summer arrives in temperate regions of the Northern Hemisphere, where about half of the world’s population lives…Different diseases have different patterns. Some peak in early or late winter, others in spring, summer, or fall…At least 68 infectious diseases are seasonal, according to a 2018 paper by Micaela Martinez of Columbia University…Some diseases have different seasonal peaks depending on latitude. And many have no seasonal cycle at all. Even for well-known seasonal diseases, it’s not clear why they wax and wane during the calendar year. “It’s an absolute swine of a field”, says Andrew Loudon, a chronobiologist at the University of Manchester. Investigating a hypothesis over several seasons can take 2 or 3 years. “Postdocs can only get one experiment done and it can be a career killer”, Loudon says. The field is also plagued by confounding variables. “All kinds of things are seasonal, like Christmas shopping”, says epidemiologist Scott Dowell, who heads vaccine development and surveillance at the Bill and Melinda Gates Foundation and in 2001 wrote a widely cited perspective that inspired Martinez’s current study. And it’s easy to be misled by spurious correlations, Dowell says.
Despite the obstacles, researchers are testing a multitude of theories. Many focus on the relationships between the pathogen, the environment, and human behavior. Influenza, for example, might do better in winter because of factors such as humidity, temperature, people being closer together, or changes in diets and vitamin D levels. Martinez is studying another theory, which Dowell’s paper posited but didn’t test: The human immune system may change with the seasons, becoming more resistant or more susceptible to different infections based on how much light our bodies experience.
…Except in the equatorial regions, respiratory syncytial virus (RSV) is a winter disease, Martinez wrote, but chickenpox favors the spring. Rotavirus peaks in December or January in the US Southwest, but in April and May in the Northeast. Genital herpes surges all over the country in the spring and summer, whereas tetanus favors midsummer; gonorrhea takes off in the summer and fall, and pertussis has a higher incidence from June through October. Syphilis does well in winter in China, but typhoid fever spikes there in July. Hepatitis C peaks in winter in India but in spring or summer in Egypt, China, and Mexico. Dry seasons are linked to Guinea worm disease and Lassa fever in Nigeria and hepatitis A in Brazil.
Seasonality is easiest to understand for diseases spread by insects that thrive during rainy seasons, such as African sleeping sickness, chikungunya, dengue, and river blindness. For most other infections, there’s little rhyme or reason to the timing. “What’s really amazing to me is that you can find a virus that peaks in almost every month of the year in the same environment in the same location”, says Neal Nathanson, an emeritus virologist at the University of Pennsylvania Perelman School of Medicine. “That’s really crazy if you think about it.” To Nathanson, this variation suggests human activity—such as children returning to school or people huddling indoors in cold weather—doesn’t drive seasonality. “Most viruses get transmitted between kids, and under those circumstances, you’d expect most of the viruses to be in sync”, he says.
…A 2018 study in Scientific Reports supports the idea. Virologist Sandeep Ramalingam at the University of Edinburgh and his colleagues analyzed the presence and seasonality of nine viruses—some enveloped, some not—in more than 36,000 respiratory samples taken over 6.5 years from people who sought medical care in their region. “Enveloped viruses have a very, very definite seasonality”, Ramalingam says.
RSV and human metapneumovirus both have an envelope, like the flu, and peak during the winter months. None of the three are present for more than one-third of the year. Rhinoviruses, the best-known cause of the common cold, lack an envelope and—ironically—have no particular affinity for cold weather: The study found them in respiratory samples on 84.7% of the days of the year and showed that they peak when children return to school from summer and spring holidays. Adenoviruses, another set of cold viruses, also lack an envelope and had a similar pattern, circulating over half the year. Ramalingam’s team also studied the relationship between viral abundance and daily weather changes. Influenza and RSV both did best when the change in relative humidity over a 24-hour period was lower than the average (a 25% difference). “There’s something about the lipid envelope that’s more fragile” when the humidity changes sharply, Ramalingam concludes.