Many diseases wax & wane with the seasons – will COVID-19?

Written by Jon Cohen on April 9, 2020. Posted in Current News

On a December afternoon, 13 days before the winter solstice, six men and women checked into the Surrey Clinical Research Facility, part of the University of Surrey in the United Kingdom.

After having their noses swabbed to check for 16 different respiratory viruses, they walked into their own temperature-regulated rooms and, for 24 hours, each person stayed in a semirecumbent position in dim light.

Nurses placed a cannula into a vein of each person’s arm, allowing easy sampling of blood that flowed through a tube to portals in the wall. The six subjects could press buzzers for bathroom breaks, where the stool and urine were collected, but otherwise, they were alone in the near-dark.

None of these people were sick. And although the shortest day of the year was approaching, their ritual had nothing to do with pagan rites, Yuletide traditions, or the annual hippie gathering at nearby Stonehenge to celebrate the rebirth of the Sun.

Instead, they were paid volunteers in a study led by infectious disease ecologist Micaela Martinez of Columbia University to investigate 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.

U.S. President Donald Trump has expressed that hope repeatedly. “There’s a theory that, in April, when it gets warm—historically, that has been able to kill the virus,” Trump said on 14 February. But what’s known about other diseases doesn’t offer much support for the idea that COVID-19 will suddenly disappear over the next few weeks.

Different diseases have different patterns. Some peak in early or late winter, others in spring, summer, or fall. Some diseases have different seasonal peaks depending on latitude. And many have no seasonal cycle at all.

So no one knows whether SARS-CoV-2 will change its behavior come spring. “I would caution over-interpreting that hypothesis,” Nancy Messonnier, the point person for COVID-19 at the U.S. Centers for Disease Control and Prevention, said at a press conference on 12 February.

If the seasons do affect SARS-CoV-2, it also could defy that pattern in this first year and keep spreading, because humanity has not had a chance to build immunity to it.

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.

Beyond the urgent question of what to expect with COVID-19, knowing what limits or promotes infectious diseases during particular times of year could point to new ways to prevent or treat them. Understanding seasonality could also inform disease surveillance, predictions, and the timing of vaccination campaigns. “If we knew what suppressed influenza to summertime levels, that would be a lot more effective than any of the flu vaccines we have,” Dowell says.

MARTINEZ BECAME interested in seasonality when, as an undergraduate at the University of Alaska Southeast, she had a job tagging Arctic ringed seals, doing skin biopsies and tracking their daily and seasonal movements.

While working on her Ph.D., her focus on seasonality shifted to polio, a much-feared summer disease before the advent of vaccines. (Outbreaks often led to the closing of swimming pools, which had virtually nothing to do with viral spread.) Polio seasonality in turn made her curious about other diseases. In 2018, she published “The calendar of epidemics” in PLOS Pathogens, which included a catalog of 68 diseases and their peculiar cycles.

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 U.S. 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.

Nathanson suspects that, at least for viruses, their viability outside the human body is more important. The genetic material of some viruses is packaged not only in a capsid protein, but also in a membrane called an envelope, which is typically made of lipids. It interacts with host cells during the infection process and helps dodge immune attacks. Viruses with envelopes are more fragile and vulnerable to adverse conditions, Nathanson says, including, for example, summertime heat and dryness.

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.

Read more at www.sciencemag.org

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