In 1981, seven American children contracted measles during a visit to the same doctor’s office.
Three of the children had never crossed paths with the 12-year-old source patient. One child arrived at the office an hour after the infected boy had left.
The outbreak caused a stir. At the time, public-health authorities believed measles was transmitted via large respiratory droplets, the kind generated by phlegmy coughs, and required contact within about 1 meter of an infected person.
So ingrained was this belief that a major medical journal, Pediatrics, deemed the outbreak an outlier, concluding that for measles, “airborne spread is unusual.”
Of course, today we know the opposite is true. Microscopic measles particles can remain airborne and infectious for up to 2 hours and can drift far and wide. In one case, an infected athlete transmitted the disease to spectators 100 feet (30.5 meters) away. The notion that measles is primarily contracted through contact with large droplets, rather than via tiny, inhaled aerosols, has been thoroughly debunked.
One year into the Covid-19 pandemic, that same theory has been debunked with respect to SARS-CoV-2 transmission, though infection-control measures have lagged behind the science.
In one regard, the evidence supporting aerosol transmission for Covid-19 is actually stronger than it is for measles: Viable SARS-CoV-2 has been captured via air sampling, a feat that has yet to be achieved with the measles virus.
In fact, only one study, published in 2016, long after experts declared measles airborne, has captured measles RNA in the air — a study its authors called “the first study to directly detect evidence of airborne transmission of measles.” Yet in that study, testing in cell cultures failed to detect viable measles virus.
By contrast, at least six air-sampling studies have isolated SARS-CoV-2 RNA. And one, conducted at the University of Florida, proved SARS-CoV-2 viral particles — captured as far as 4.8 meters from a Covid-19 patient — were viable.
“If this isn’t a smoking gun, then I don’t know what is,” asserts Linsey Marr, PhD, a Virginia Tech aerosol scientist who was not involved in the study.
Marr calls the results “unambiguous evidence that there is infectious virus in aerosols.”
The Florida study, piled atop volumes of other evidence pointing to aerosol transmission, has intensified calls for more robust infection control indoors — in hospitals, nursing homes, dental practices, and retail establishments.
With ultra-contagious SARS-CoV-2 variants now surging globally, the stakes could not be higher.
“It is very clear that aerosols play a considerable role in the transmission of Covid-19 and that we are unlikely to prevail against this pandemic unless we acknowledge that fact,” asserts Justin Morganstern, M.D., a Canadian emergency physician, in an evidence review.
While physical distancing and masks remain important, Morganstern argues, “We should be looking at the extra precautions we can add to stem the spread of this disease.”
Foremost among these precautions should be air filtration and dis-infection, say experts, including Kevin Fennelly, M.D., of the U.S. National Institutes of Health.
At hospitals and nursing homes, infection-control protocols are based on “old data and inferences,” Fennelly asserts in The Lancet Respiratory Medicine. Droplet transmission is not driving the pandemic, he argues, and precautions should be updated to “account for the predominance of small particles within infectious aerosols.”
Coronavirus in the Air
At the pandemic’s outset, health authorities made the same assumption about SARS-CoV-2 that they’d made, erroneously, about measles in the 1980s and tuberculosis in the 1950s: that aerosol transmission, if it happened at all, was “probably very rare.”
But that assumption soon began to wither.
Quickly, it became clear that asymptomatic carriers were spreading Covid-19 in huge numbers, without sneezing or coughing.
What’s more, scientists identified outbreaks — on cruise ships and bus rides, at choir practices and ski resorts, in call centres, restaurants, and shopping malls — that could not be explained by surface or droplet transmission.
Strengthening the case for aerosol spread, scientists captured SARS-CoV-2 genetic material on surfaces that patients could not possibly have touched, such as air outlet vents and air-handling grates.
Even more compelling, coronavirus particles were captured in the air — above flushing toilets, in hospital nurses’ stations and changing rooms, in hallways outside patient rooms, and inside patient rooms beyond 6 feet from the patients.
Still, questions persisted: Was the RNA viable? Could the captured particles actually invade a cell, replicate, and trigger infection? Or were they inert, harmless fragments of genetic material?
The answer was elusive because aerosols, microscopic and fragile, are easily damaged by the air-sampling process.
But the University of Florida team used new, more sophisticated technology, preserving SARS-CoV-2 RNA captured in the air 15 feet from a Covid-19 patient. The genome sequence of the collected virus matched the sequence isolated from the patient.
The study, says lead researcher John Lednicky, PhD, proved “conclusively” that viable SARS-CoV-2 particles, small enough to be inhaled, can linger in the air and pose a risk to those in the vicinity.
The study squelched doubt that Covid-19 can spread — and readily — via aerosols.
Part two coming soon.