Air travel spreads the virus. People with Covid-19 travel from one place to another, bringing the virus with them. That may not matter much when the virus is already spreading within the community at the destination and they’re beyond the containment stage. But early in the pandemic it’s how the virus traveled from one place to another.
That’s different from the extent to which the virus spreads from person to person while on the aircraft. We’ve known since early on that it happens, but it’s rare, primarily because of hospital grade air filtration.
However, there’s reason to believe that there has been some spread on three specific flights:
- On January 24, 2020 the Singapore Airlines Singapore to Hangzhou flight 100 passengers had visited Wuhan five days earlier. Several were infected with the virus. It appears at least one person contracted the virus on the trip.
- March 1 Vietnam Airlines London Heathrow – Hanoi. This is the first flight identified, and the one with the most spread, that I’ve talked about for months frequently when writing about risk of Covid spread on planes (such as here, here) and here).
- March 9 Cathay Pacific Boston – Hong Kong, a couple who had contracted the virus may have given it to one flight attendant who gave it to another.
Often when we point to people who supposedly ‘got the virus on planes’ it turns out to be just as likely that they were exposed to the virus in the gate area or jetway. That is an importance difference because it suggests handling the issue in different ways, and tells us something about where risk is and what tactics are protective.
The CDC last week released a study declaring risk of inflight transmission of the virus, based on an investigation of the London Heathrow – Hanoi flight that we’ve known about for months. This isn’t new, despite it hitting the news broadly now.
Their caution is that as more long haul flights take place the virus could spread more. Singapore – Hangzhou is the length of a cross country flight in the U.S., so if the virus spread on that flight then ultra long haul isn’t necessary for spread. And they don’t explain why HEPA filtration would work less well on long flights. Longer exposure might create more close interactions but it isn’t obvious that’s the case either.
And much less reported is that the study notes no spread from symptomatic individuals on a 15 hour flight from China to Canada, and flights from China to France and Thailand. The lack of inflight spread from infected individuals is less telling, though, than it might be because most people who catch the virus don’t spread it. A small number, perhaps 10% of cases, are responsible for 80% of spread.
Nonetheless documenting just a handful of cases where inflight spread may have occurred is significant as an argument for the safety of flying because there’s never been a virus studied as extensively and intensively in a short period of time as this one. There are millions of people flying throughout the world week after week during the global pandemic, yet we haven’t been able to identify more flights where the virus spread (though there are probably some). This is highly suggestive that inflight spread is very rare.
Contrast that with recent CDC findings that spread of Covid-19 is linked heavily to dining in restaurants. Indoor activities are conducive to virus spread because the virus becomes aerosolized and viral loads build up the longer an individual shedding the virus spends in the enclosed environment. Restaurants generally lack the filtration and outside air exchange of planes, which is why air travel is much safer than indoor dining.
Meanwhile the most common response to the CDC-promoted study of that London – Hanoi flight is that it (and other identified flights where spread may have occurred) is that they were pre-mask wearing. That’s true, and 50% effective masks can reduce viral load exposure 10-fold. I think the better argument is the rarity of spread, which is consistent with the finding that airline employees have lower Covid rates than the general population despite repeated exposure across a large number of flights.
In reporting this new study, what the news isn’t telling you is how much of an outlier spreading events on planes actually are.