A group representing airlines has reviewed studies which it says show only
44 cases of Covid-19 are reported to have been caught on flights, leading an epidemiologist to question its findings.
Over the same period some 1.2 billion passengers have travelled although the International Air Transport Association concedes studies could not pick up every case.
"The risk of a passenger contracting Covid-19 while on board appears very low. With only 44 identified potential cases of flight-related transmission among 1.2 billion travellers, that's one case for every 27 million travellers.''
The association is a lobby group which represents most of the world's airlines.
''But even if 90 per cent of the cases were unreported, it would be one case for every 2.7 million travellers. We think these figures are extremely reassuring,'' said David Powell, IATA's medical adviser.
New Zealand epidemiologist Michael Baker said he was skeptical of the low numbers the survey of studies had produced.
''Everything we know about Covid is that it is transmitted by close contact in indoor environment. You would have to assume that flights are a high-risk environment,'' he said.
''You have to be very skeptical of the evidence presented by IATA and I don't think its terribly responsible to say that we've got 44 cases of Covid reported and the same breath say we've had 1.2 billion passengers. Of course that is not how you would calculate the level of risk.''
The origin of millions of cases of Covid around the world had not been determined.
He said the airline industry was very good traditionally in assessing safety risk but it needed to seek more independent advice on assessing danger around Covid.
IATA's Powell said the vast majority of published cases occurred before the wearing of face coverings inflight became widespread.
The association's director general and chief executive Alexandre de Juniac says the risk of contracting the virus on board aircraft appears to be in the same category ''as being struck by lightning''.
(His struck-by-lightning-comparison of the one in 27 million odds in fact overstates the risk, with the US National Weather Service calculating are closer to one in a million in any given year although the odds over a lifetime are one in 15,300)
"There is no single silver-bullet measure that will enable us to live and travel safely in the age of Covid-19. But the combination of measures that are being put in place is reassuring travellers the world over that Covid-19 has not defeated their freedom to fly. Nothing is completely risk-free,'' said de Juniac.
The new figures come from a joint publication by Airbus, Boeing and Embraer of separate computational fluid dynamic (CFD) research conducted by each manufacturer in their aircraft.
While methodologies differed slightly, each detailed simulation confirmed that aircraft airflow systems do control the movement of particles in the cabin, limiting the spread of viruses.
Data from the simulations yielded similar results:
• Aircraft airflow systems, High Efficiency Particulate Air (Hepa) filters, the natural
barrier of the seatback, the downward flow of air, and high rates of air exchange
efficiently reduce the risk of disease transmission on board in normal times.
• The addition of mask-wearing amid pandemic concerns adds a further and significant extra layer of protection, which makes being seated in close proximity in an aircraft cabin safer than most other indoor environments.
The role of filters has been backed by David Nabarro, WHO special envoy for Covid-19, who recently said that air travel is "relatively safe" when it comes to the spread of coronavirus.
"So the one good thing about aeroplanes is that the ventilation system includes really powerful filters which means that in our view they are relatively safer," he told BBC News.
In this country Air New Zealand says none of its crew has tested Covid-19 positive since early April - in any setting - and this is after thousands of flights. While crew aren't necessarily exposed to the same extent as passengers on planes, they flew far more frequently.
The airline's chief medical officer Ben Johnston said the IATA review backed up previous research with more evidence around the importance of air flow and protection from seat backs.
While the airline's regional turbo-prop aircraft didn't have Hepa filters, which could not be retrofitted, global studies showed air flow, and cabin layout meant any infection was also extremely low. There were fewer passengers on board and the flights were usually of short duration, he said.
The case of a Covid infected passenger on a Christchurch-Auckland flight last month passing it on to others while on board was inconclusive as transmission may have been at other stages of the journey.
Johnston said Air New Zealand was part of a big study with health authorities using contact tracing data aboard its planes since February.
While the IATA survey concentrated on only the plane journey, Baker said it was necessary to assess risk around all aspects of travel including the journey to and through airports.
He said the Government and Air NZ had missed an important opportunity to enforce more strict mask use on planes now alert levels had dropped as they had been shown to be effective in stopping the spread of the disease.
IATA says its data collection, and the results of the separate simulations, align with the low numbers reported in a recently published peer-reviewed study by Freedman and Wilder-Smith in the Journal of Travel Medicine.
''Although there is no way to establish an exact tally of possible flight-associated cases, IATA's outreach to airlines and public health authorities combined with a thorough review of available literature has not yielded any indication that onboard transmission is in any way common or widespread.''
Studies showed the most serious case of spread was on a London-Hanoi flight in which there were 15 probable secondary infections. A Sydney-Perth flight in March resulted in eight definite and three probable infections on board, confirmed by genome sequencing.
The Freedman/Wilder-Smith study points to the efficacy of mask-wearing in further reducing risk.
Mask-wearing on board was recommended by IATA in June and is a common requirement on most airlines. In this country the Air Line Pilots Association pushed for mask use in May.
Some go further with Qatar requiring passengers to wear face shields too.
Johnston said the research didn't back going that far as they were most useful in medical settings and there was a risk of passengers becoming resistant to increasingly stringent measures.
Under alert level 1 on domestic flights mask wearing not compulsory but is still encouraged for crew. Passengers were ''free to wear them.''
International Civil Aviation Organisation (ICAO) guidance adds layers of
protection on top of the airflow systems which already ensure a safe cabin environment with very low risks of inflight transmission of disease.
Mask-wearing is one of the most visible but managed queuing, contactless processing, reduced movement in the cabin, and simplified onboard services such as reduced food and beverages were helping cut risk.