By Michael Le Page
Shutterstock / Sopotnicki
Is it safe to fly with the coronavirus still circulating? That depends partly on where you are. But while hard evidence is scarce, it appears the risk of being infected with covid-19 during a flight is relatively low.
“Overall, planes are probably safer than poorly ventilated pubs, where similar densities of people do not wear masks and talk a lot and loudly,” says Julian Tang at the University of Leicester in the UK.
It is of course safest not to travel, especially if you are vulnerable. And if you have symptoms that might be coronavirus, you definitely must not travel.
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Walking, cycling or travelling in your own vehicle minimises the risk of coming into contact with people who might be infected. If you do need to use public transport, the risk depends firstly on the odds of an infected person being on the same bus, train or plane, and then of them passing the virus on to you.
Travelling in South Korea, for instance, where just 1 in about 225,000 people test positive every day, is inherently far safer than travelling in the US, where 1 in 6500 people test positive every day. In the UK, 1 in 60,000 people are confirmed positive daily.
If you do end up on a plane near someone who is infected, how likely are you to be infected? We just don’t know for sure because there is little to go on besides what we know about how viruses spread in general, and a few case studies on the spread of the coronavirus.
One case study, for instance, describes a 5-hour flight from Singapore to China on 23 January, where 11 of the 325 people on board were infected by one man. Passengers were screened before boarding, but the man developed a fever during the flight and was not wearing a mask. It is not clear how transmission occurred.
However, when an infected couple flew from China to Canada on 22 January, none of the other 350 passengers on the 15-hour flight were infected. Masks were worn.
In a document on the medical evidence for in-flight transmission, the International Air Transport Association (IATA), an industry body, says that when four airlines followed up with 1100 passengers confirmed to be infected after flying, just one possible case of in-flight transmission was identified.
It’s not clear how reliable this finding is, as no details have been published and IATA did not respond to queries. Nonetheless, there are reasons to think the risks are low.
Many airports check people’s temperatures before they board, and airlines now disinfect planes between flights and require passengers to wear masks. The air on planes is also replaced every three to five minutes, and the air that is recirculated goes through HEPA filters that should remove almost all droplets containing viruses.
“The ventilation systems on planes are very effective in reducing the overall concentration of any airborne pathogen exhaled by passengers,” says Tang. The main risk may be face-to-face conversations where air can be exchanged before being pulled away – along with any conversations before or after the flight.
Arnold Barnett at MIT has estimated that when flying in the US, the risk of infection is about 1 in 4000 if the flight is full. If the middle seats are left empty, the risk falls to 1 in 8000.
For the UK, the equivalent risks are about 10 times lower, Barnett says. That is, there is just a 1 in 40,000 chance of infection. These are just rough estimates based on many assumptions, however.
It is not clear if travelling on trains or buses is more or less risky. While ventilation systems on trains and buses vary considerably, though, at least some are less effective than those on planes. For instance, the air on Eurostar trains is replaced only every 15 minutes. Eurostar did not answer when asked if its trains have HEPA filters.
In China, a study of 2334 people who travelled on high-speed trains while infected concluded they infected 234 other passengers – but there were 72,000 people who sat within three rows of those already infected, and just 234 of them got infected too.
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