The coronavirus is not the flu. But the two viruses have something crucial in common: Both have been described as spreading primarily through close contact with symptomatic people or the surfaces they’ve touched.
Mounting evidence may be starting to turn the tide on that message. Last week, the World Health Organization modified its stance on coronavirus transmission, acknowledging that the virus may also hop from person to person by lingering in the air, trapped inside tiny aerosols that can traverse the length of room.
A wealth of evidence has shown the same is true of flu viruses, which also attack cells in the human airway. Researchers have even isolated infectious flu viruses from exhaled breath. But examining the relative contributions of different modes of transmission — whether through contaminated surfaces; tiny aerosols; or large, liquid-laden droplets expelled by coughs or sneezes — remains a daunting task for experts in the field.
“We’ve been studying the flu for 102 years and still don’t know for sure how it’s transmitted,” said Dr. Don Milton, an environmental health researcher and aerosol transmission expert at the University of Maryland.
A new study from Dr. Milton and his team, published Monday in PLoS Pathogens, contains some of the latest data supporting an aerosol route for the flu. The researchers deliberately infected 52 volunteers with a strain of influenza, a setup called a human challenge trial. They then had these “donors” mix and socialize with 75 healthy “recipients” for four days to test how often the virus got passed to others.
Under these conditions, Dr. Milton and his colleagues expected the virus to spread to several of the recipients. That was certainly the case when his colleagues, also authors on the new paper, ran a pilot study in another group of volunteers under similar conditions in 2009: After just two days of bunking up, about 8 percent of the recipients were infected.
In this new experiment, the researchers doubled the number of days the study participants were allowed to mingle. But they found only one new case of flu among the 75 volunteers who hadn’t been intentionally dosed with the virus — an infection rate of just 1.3 percent.
The biggest difference between the two experiments, Dr. Milton said, was their backdrop. In its first iteration, the study took place in poorly ventilated hotel rooms; in its second, the researchers housed their participants in a research facility with much improved air flow. In the new study, “we had at least seven times more ventilation per person,” he said.
A multitude of factors, such as past exposures to infection, can influence a person’s vulnerability to a virus. But “the fact that they only had one transmission event, compared to what the pilot study suggested, indicates that ventilation is important,” said Linsey Marr, an aerosols expert at Virginia Tech who wasn’t involved in the study.
While good ventilation can dilute aerosols, it is far less effective against droplets, which are much wider and heavier — in the same way that a passing breeze would perturb the trajectory of a Ping-Pong ball, but not a cannonball.
The study points to a more “important role” for aerosolized flu transmission than some might assume, Dr. Marr said.
Determining the exact size of that role, however, is another matter entirely. “It’s very hard to conduct these human challenge studies and separate the different modes of transmission,” Dr. Marr said. That problem applies across respiratory viruses, including the coronavirus.
Part of the problem is the continuum on which aerosols and droplets exist. Though they go by different names, the two categories really belong to the same group: globs of fluid that come in varying sizes. Blobs less than five micrometers in diameter are termed aerosols, which can exit the airway at the slightest breath and waft away; anything larger is a droplet, hefty enough to fall to the ground within a few feet of its source. The boundary between them is somewhat arbitrary, though generally speaking, the smaller the particle, the farther it travels.
When people expel fluid from their airway, it tends to manifest in a mixture, some bigger, some smaller and everything in between, said Seema Lakdawala, who studies influenza transmission at the University of Pittsburgh.
Even after they exit an individual, these fluidic blobs remain dynamic. Large droplets, for instance, can disperse or evaporate into little aerosols in midair. Others might scatter onto a surface or a hand, lingering for minutes or hours before encountering someone new. And the rates at which all these events occur can shift, depending on the force with which someone, maybe a loud talker, expels these droplets or the amount of air flow in an area, Dr. Lakdawala said.
“Everyone thinks transmission is a very binary concept,” she added. “The reality is that there is a continuum of aerosols.”
Still, researchers have come up with some truly innovative experiments to investigate transmission for certain viruses in the past, Dr. Marr said.
In the 1980s, researchers in Wisconsin dosed a group of male volunteers with rhinovirus, which causes the common cold, and sat them down for 12 hours of poker with their healthy counterparts. Some of the recipients, who were susceptible to new infections, were given large, plastic collars or arm restraints that kept them from touching their faces. In this setting, aerosols and droplets were likely to have been the most feasible route of transmission, Dr. Marr said. Though the study was small, its participants seemed to be infected at similar rates, regardless of whether they’d been restrained.
That experiment probably couldn’t be conducted today, Dr. Marr said. But perhaps a variant of it could help tease out some of the dynamics of flu or coronavirus transmission.
These studies grow more pressing as researchers home in on how long the coronavirus can persist while adrift in the air. In March, a study in the New England Journal of Medicine suggested that, under ideal laboratory conditions, the virus remained viable for up to three hours in aerosols; a more recent study, led by Chad Roy of Tulane University, found the germ’s longevity might be even more impressive.
There’s still a long way to go before scientists fully understand exactly how and when the coronavirus most easily spreads, Dr. Marr said. But recent events appear to have breathed new life into the study of aerosol transmission.
“I’ve been studying this for more than a decade,” she said. “In the last four months, I’ve seen a greater willingness to consider the importance of aerosols than I’ve seen in the last 12 years.”