Science - USA (2020-03-20)

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20 MARCH 2020 • VOL 367 ISSUE 6484 1295

CREDITS: (GRAPHIC) N. DESAI/

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; (DATA) PROJECT TYCHO

sons 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 shop-

ping,” says epidemiologist Scott Dowell, who

heads vaccine development and surveillance

at the Bill & 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. In-

fluenza, 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 par-

ticular times of year could inform disease

surveillance, predictions, and the timing of

vaccination campaigns. It might even point

to new ways to prevent or treat them. “If we

knew what suppressed influenza to summer-

time levels, that would be a lot more effec-

tive than any of the flu vaccines we have,”

Dowell says.

MARTINEZ BECAME interested in seasonal-

ity when, as an undergraduate at the Uni-

versity of Alaska Southeast, she had a job

tagging Arctic ringed seals, doing skin biop-

sies, 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 ad-

vent of vaccines. (Outbreaks often led to the

closing of swimming pools, which had vir-

tually 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, respira-

tory syncytial virus (RSV) is a winter dis-

ease, 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 mid-

summer; gonorrhea takes off in the summer

and fall, and pertussis has a higher inci-

SCIENCE

Dec. Jan. Feb.Feb. Apr.Mar. Apr.Mar. MayMay Jul.Jun. Jul.Jun. Aug.Aug. Sep.Sep. OcOct. Nov.Nt. ov.

Winter Spring Summer Fall

Smallpox

California

1937

Rubella

California

1971

Mumps

California

1973

Hepatitis A

California

1980

Pertussis

California

1945

Infuenza

United States

1934

Measles

United States

1933

Chickenpox

United States

1975

Polio

United States

1952

Most cases in the year

The calendar of epidemics

At least 68 infectious diseases are seasonal, according to a 2018 paper by Micaela Martinez of Columbia

University. But they’re not in sync, and seasonality varies by location. Here, each bubble represents the

percentage of annual cases that occurred in each month. (The data are old because many diseases declined—

in some cases to zero—after the introduction of vaccines.)