• SHARE

Is the coronavirus an airborne pathogen or not?

While Japan and many other countries have shaped their policies based on the premise that the virus can be transmitted by heavy droplets at close range, questions remain over how far those droplets can travel to infect others and whether or not smaller, more mobile droplets can spread the virus.

Nearly two years since the first COVID-19 cases were reported in Wuhan, China, there is a growing consensus among the world’s scientists that the virus that causes the deadly disease is more airborne than many originally thought.

In order to reduce the spread of the virus, Japan has enacted measures to reduce business hours for restaurants and urged people to avoid crowded and close-contact settings, particularly those that are indoors with poor ventilation. The government has also urged people to wear masks and wash their hands.

But if the virus truly is transmitted by tiny particles and over larger distances, the effectiveness of those measures may be limited. Japan has not yet officially acknowledged the possibility of such airborne transmission.

At present, there are two focal points in the discussion. The first is getting policy-level clarity on the distinction between “aerosol viral transmission” and an “airborne virus.” The second is whether the virus can be transmitted in aerosol form, or as “droplet nuclei,” over larger distances.

Since the start of the pandemic in early 2020, Japan has focused mainly on taking precautions against transmission through droplets and contact with contaminated surfaces and emphasized taking measures against the
Since the start of the pandemic in early 2020, Japan has focused mainly on taking precautions against transmission through droplets and contact with contaminated surfaces and emphasized taking measures against the “Three Cs”: closed spaces with poor ventilation, crowded places and close-contact settings. | BLOOMBERG

The government’s perspective

The Health, Labor and Welfare Ministry acknowledges aerosols that linger in the air and travel more than 1 meter can play a role in COVID-19 infections, especially in crowded settings, but it does not consider such particles as being airborne.

That is because the government draws its own distinction between aerosols and droplet nuclei — something not done by the wider scientific community — and defines an airborne disease as requiring the latter, which in the official view are smaller than 5 micrometers in diameter and can travel much farther than aerosols, as a vehicle for transmission.

Infections through droplet nuclei, as opposed to large droplets, have traditionally been considered possible for only highly transmissible diseases such as tuberculosis and measles.

“The scientific knowledge (on COVID-19) has not been determined regarding whether it can be transmitted via droplet nuclei,” a health ministry official said, adding that the government does not know for certain whether airborne transmission can occur because it believes scientific discussions on it have not yet reached a conclusion.

The official added that the World Health Organization and the U.S. Centers for Disease Control and Prevention (CDC) similarly acknowledge possible aerosol infection but do not use the term “airborne transmission” for COVID-19.

Heavier droplets, such as respiratory droplets which are larger than 5 micrometers in diameter, typically drop to the ground between 1 to 2 meters within seconds, meaning infection can only be spread among people in relatively close range of each other. Aerosols, on the other hand, can range from 0.001 to 100 micrometers, which makes the topic of infection range far more complicated. One micrometer is equal to a thousandth of a millimeter.

If the coronavirus truly is transmitted by tiny particles and over larger distances, the effectiveness of Japan's measures may be limited. | BLOOMBERG
If the coronavirus truly is transmitted by tiny particles and over larger distances, the effectiveness of Japan’s measures may be limited. | BLOOMBERG

For this reason, the health ministry has focused on infections via large droplets or respiratory droplets, which can infect people within 2 meters of a person coughing or sneezing.

Another key infection route the ministry has focused on is when an infected person coughs or sneezes on their hands and then touches objects around them, allowing the virus to transmit to others via door knobs, light switches or other surfaces — a process known as “fomite” transmission.

The government’s COVID-19 subcommittee has taken a similar stance.

Shigeru Omi, who chairs the subcommittee, has explained that experts on the government’s advisory panel believe that aerosol transmission is limited to situations that fall under the “Three Cs”: closed spaces with poor ventilation, crowded places and close-contact settings.

“Airborne transmission has been confirmed in some viruses such as tuberculosis or measles where much smaller particles than microdroplets (aerosols) float in the air for a long period through air conditioning and cause infections at a long distance,” Omi told reporters in late July. “We on the government’s expert panel and the health ministry’s advisory board do not think this is what’s happening (with the coronavirus). If that’s the case, it would have spread more widely in an instant.”

Kazuhiro Tateda, a Toho University professor and a distinguished infectious disease scholar who serves on the government’s expert panel, has commented publicly that the possibility of such airborne infection has not yet been proven.

Shigeru Omi, chairman of the government's coronavirus subcommittee, in June | POOL / VIA REUTERS
Shigeru Omi, chairman of the government’s coronavirus subcommittee, in June | POOL / VIA REUTERS

Scientists’ perspective

But those views stand in stark contrast with a growing consensus among the world’s scientists that airborne transmission may be the main method of transmission for COVID-19.

In an article in the cutting-edge research journal Science that reviewed more than 200 studies on the coronavirus, researchers stated that droplet and fomite transmission alone cannot account for the number of superspreader events — where people are infected en masse and over long distances — and that airflow simulations provide strong and unequivocal evidence of airborne transmission.

For a long time after the pandemic began, the health ministry had listed only inhalation of droplets or fomite transmission via direct contact with an infected person’s eye, nose or mouth as possible reasons for coronavirus infections on its website’s Q&A section on COVID-19.

And it was not until around September that it added aerosols as one of the infection possibilities, the health ministry official said.

“The fact that the health ministry listed aerosols is a good thing but there’s still a problem,” said Tsuyoshi Hondo, an associate professor at Tohoku University’s graduate school of sciences. “That’s because although the aerosol infection is an airborne infection, the health ministry doesn’t regard it as such.”

The discrepancy may suggest that the health ministry requires diseases officially classified as airborne to be highly transmissible, but according to Hondo, as long as the health ministry is preoccupied with such a distinction, the debate can theoretically go on forever.

A subway train in Tokyo in July 2020. Experts say the key to fighting COVID-19 is a constant ventilation system. | REUTERS
A subway train in Tokyo in July 2020. Experts say the key to fighting COVID-19 is a constant ventilation system. | REUTERS

It’s true that the COVID-19 virus’s reproduction number, which measures how transmissible a virus is, may be lower than that of measles. But experts say that doesn’t mean that it is not an airborne pathogen, adding that there is robust evidence suggesting airborne transmission in many viruses in the coronavirus family, including the one that causes severe acute respiratory syndrome, or SARS.

Hondo served as one of the coordinators for 38 Japanese scientists who issued an emergency statement in August calling on the government to take measures based on the assumption that COVID-19 is airborne.

The implications of airborne transmission are profound. Since the start of the pandemic in early 2020, the country has focused mainly on taking precautions against droplet and fomite transmission and emphasized taking measures against the Three Cs.

COVID-19 response

Acknowledging COVID-19 is capable of airborne transmission would call for a drastic revision on Japan’s policies, such as shifting the focus even further toward ventilation.

In addition, Japan would have to consider a stricter masking policy to be in line with the recommendations of the 38 scientists, who are calling for the adoption of single-use disposable nonwoven masks that more closely fit over people’s faces, rather than the cloth or polyurethane alternatives that can’t filter out virus particles that are 5 micrometers or smaller.

Germany, for example, in January made the use of N95 and other medical masks mandatory for people using public transportation and at supermarkets.

The health ministry remains ambivalent about whether the coronavirus can be transmitted via the inhalation of droplet nuclei, particles smaller than 5 microns in diameter that can travel much farther than aerosols. | REUTERS
The health ministry remains ambivalent about whether the coronavirus can be transmitted via the inhalation of droplet nuclei, particles smaller than 5 microns in diameter that can travel much farther than aerosols. | REUTERS

Instead of imposing measures to curb people’s movements or consider strict lockdowns, the scientists are calling on Japan to immediately take the measures that have been proven to reduce the risk of infections, including making surgical masks mandatory in public places.

Given the broader scientific consensus and the example of other nations, why is the Japanese government unwilling to say that the coronavirus is an airborne pathogen?

That reluctance may be because doing so would suggest that its anti-prevention measures so far were not effective, said Masahiro Kami, executive director of the Medical Governance Research Institute, a Tokyo-based nongovernmental organization.

During multiple states of emergency, the government forced many restaurants and bars to stop serving alcohol, but Kami says that measure may have been pointless because the risk of infection is not limited to when people are drinking in close proximity.

“There’s no reason to target only restaurants,” he said. “The Three Cs measures are also superfluous because some studies have shown that erecting plastic partitions, for example, impede air flow and actually make the environment more susceptible for infection. The article in Science means that it’s a scientific consensus, and the stance by the government’s advisory board stands in the complete opposite to the global consensus.”

Kami emphasized the importance of monitoring carbon dioxide levels, which shows how ventilated a room is, to mitigate indoor infections. As a measure against the coronavirus, the central government calls on businesses to try to keep the concentration of atmospheric carbon dioxide inside rooms at 1,000 parts per million or lower, which compares with around 420 ppm in clean outside air.

The streets of Tokyo in October | BLOOMBERG
The streets of Tokyo in October | BLOOMBERG

Experts say key aspects include a constant ventilation system and the widespread use of air cleaners equipped with high-efficiency particulate air, or HEPA, filters in order to effectively reduce airborne COVID-19 particles.

The government has also provided subsidies for installing carbon dioxide monitoring devices, which Kami claims should become ubiquitous.

“For example, a packed subway (car) can be 1,600 to 1,800 ppm (of carbon dioxide), but when you open the windows, the levels fall drastically,” he said. “If each car is equipped with a carbon dioxide monitor, passengers start opening the windows voluntarily to protect themselves if the levels are high or move to another car that is less crowded.”

In a time of both misinformation and too much information, quality journalism is more crucial than ever.
By subscribing, you can help us get the story right.

SUBSCRIBE NOW

PHOTO GALLERY (CLICK TO ENLARGE)