Study Shows That Measles Can Spread Beyond Those Seated Immediately Around An Infectious Person On Aeroplanes
New research published at this week’s Annual Scientific Meeting of the Australian Society for Infectious Diseases (ASID) in Canberra, Australia, demonstrates that direct contact tracing of aeroplane passengers seated immediately around an infected person, as is currently recommended in Australian and other international public health guidelines, is not an effective strategy to prevent further cases. The research is by Dr Gary Dowse, Communicable Disease Control Directorate, Department of Health, Perth, WA, Australia, and colleagues.
Not much data exists regarding measles transmission during aeroplane travel, or the effectiveness of contact tracing. In this study, Dowse and colleagues analysed the risk of transmission associated with infectious measles cases who travelled on flights to or within Australia. The authors obtained information from all state and territory health authorities in Australia on measles cases notified in Australia in the period Jan 2007 to June 2011, and who were likely to have been infectious or infected while travelling on aeroplanes.
The researchers identified 45 infectious cases who had travelled on aeroplanes, involving 49 separate flights (some had connecting flights). 20 secondary infections (people who were infected by the primary cases and became ill in the 10-14 days after the flight) occurred in people on 7 of 49 flights on which infectious cases travelled. Secondary cases occurred on 7 of 36 international flights and none of the 13 domestic flights that infectious cases had travelled on.
Most of the primary cases were Australians who were infected overseas, in a range of countries – primarily developing countries in SE Asia and Asia where measles transmission remains endemic, but also in some European countries, including the UK, which have had measles epidemic activity in recent years. Almost all the secondary cases were also Australian..
Nine (45%) of the secondary cases were seated within 2 rows of the index case, while 11 cases (55%) were seated outside 2 rows, 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. “Because of delays in diagnosis and notification of cases, and the additional time to access flight information, only 31% of flight manifests were available to health authorities within 5 days of travelling, ” says Dr Dowse. “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.” The delays identified do also have implications for the timeliness of being able to contact trace people potentially exposed to other infectious diseases, which may be more serious than measles..
The authors say that the results show that, despite secondary measles transmission occurring in 1 in 5 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 says: “We recommend that direct contact tracing to identify susceptible people exposed to measles cases on aeroplanes should not be undertaken routinely, and other strategies should be considered.”
He concludes other strategies could include 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, if airlines can provide such details (or send messages on behalf of health authorities if they are unable to release such information).
Source: Australasian Society of Infectious Diseases