How air travel spreads infection and why airlines should look to prevention

Ten years ago, the world faced the SARS epidemic. You may remember the images of people, many wearing surgical masks, arriving in airports and having their temperatures checked before being allowed past the arrivals hall.

A different type of coronavirus has been identified in the Middle East in recent months; thankfully it does not seem to have a high level of infectiousness. And just last week, China reported the first cases of a bird flu (H7N9), which the World Health Organisation is tracking closely.

SARS was the first disease to really highlight the risk of respiratory infection being spread between airplane passengers. We breathe recycled air when travelling in aircraft. Older aeroplanes were designed to supply fresh air through the cabin. Newer aircraft, in an effort to cut airlines’ fuel bills, use a system of recirculating air.

Actively infectious
Although rare, there have been reports of TB being transmitted to fellow passengers by an air traveller with an actively infectious form of the disease. Caused by the bacteria mycobacterium tuberculosis, TB is spread by droplets in the air.

A person with infectious TB can expel TB germs when they cough or sneeze and people in the surrounding area can then inhale these. Each person with active infectious TB infects on average 10-15 people a year.

Now Australian researchers have shown measles can be transmitted farther than had previously been thought during air travel. In a study presented last month at the Australian Society for Infectious Diseases annual meeting, Dr Gary Dowse of the Department of Health in Perth and colleagues analysed the risk of transmission associated with infectious measles cases in those who travelled on flights to or within Australia between January 2007 and June 2011.

The researchers identified 45 infectious cases involving 49 separate flights (some passengers had connecting flights). Of these, 20 secondary infections occurred in people who were infected by the primary measles cases and became ill in the 10-14 days after a flight.

Secondary cases occurred on seven of 36 international flights but on none of the 13 domestic flights.

Secondary cases
Nine (45 per cent) of the secondary cases were seated within two rows of the index case, while 11 cases (55 per cent) were seated more than two rows away. This is beyond the range of seats for which contact tracing is currently recommended.

Secondary transmission was more likely to occur when primary cases were in young children, and where there were multiple infectious cases travelling.

Somewhat worryingly, only 31 per cent of flight manifests were available to health authorities within five days of travelling.

“This means that in most cases it was too late to provide vaccine or immunoglobulin that might prevent illness in exposed passengers who were found to still be susceptible to infection,” Dr Dowse noted. The delays identified also have implications for the timeliness of being able to contact trace people potentially exposed to other, more serious infectious diseases.

The authors say the results show that, despite secondary measles transmission occurring in one in five international flights with infectious cases, the risk was not clearly related to seating proximity and contact tracing was ineffective, especially given delays in diagnosis, notification, and accessing flight manifests.

Dr Dowse said other strategies need to be considered, including using general media alerts identifying flights on which passengers may have been exposed, and which provide advice as to what passengers should do (such as see their doctor promptly if they think they have not been vaccinated as there may be time to be protected, or if they develop symptoms consistent with measles).

Another possible strategy would be to consider SMS messaging or email alerts to all passengers on an affected flight, assuming airlines can provide such details.

It's certainly food (or air) for thought as you plan this summer's holiday getaway. And might a "no hacking cough" policy be added to a non-smoking one by airlines in the years to come?

mhouston@irishtimes.com
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