Fifty-nine laboratory-confirmed cases of Covid-19 from six of the eight different health regions were linked to just one international flight into Ireland last summer - and the plane was just 17% full and some passengers wore masks.
The case is included in a newly published issue of, a journal on infectious disease surveillance, epideomiolog, prevention and control.
It looks at one flight coming into the country last summer in detail, including the seating arrangements on the flight of people who tested positive, people who had the virus but weren't picked up through testing, and those not tested.
"The flight into Ireland was 7.5 h long and had a passenger occupancy of 17% (49/283 seats) with 12 crew," it said.
"The age of the 13 flight cases ranged from one to 65 years with a median age of 23 years. Twelve of 13 flight cases and almost three quarters (34/46) of the non-flight cases were symptomatic. After the flight, the earliest onset of symptoms occurred two days after arrival, and the latest case in the entire outbreak occurred 17 days after the flight. Of 12 symptomatic flight cases, symptoms reported included cough (n = 7), coryza (n = 7), fever (n = 6) and sore throat (n = 5), and six reported loss of taste or smell. No symptoms were reported for one flight case. A mask was worn during the flight by nine flight cases, not worn by one (a child), and unknown for three."
The study also highlights how the flight into Ireland was just one link in a broader international chain of air travel.
"Thirteen cases were passengers on the same flight to Ireland, each having transferred via a large international airport, flying into Europe from three different continents," it said.
Some of those on board had spent up to 12 hours overnight in the transit lounge during the stopover, others shared a separate transit lounge and two groups had separate short waits of under two hours in the general airport departure area.
The source case is still not known but the chain of transmission after the plane landed here resulted in 59 cases, driven by social gathering.
In addition, one passenger declined later testing, and the remaining 11 passengers were not contactable. "No data were available for these 11 passengers or for the crew with regard to symptomatology and subsequent illness," it said.
According to the study: "This outbreak demonstrates the potential for spread of SARS-CoV-2 linked to air travel. Onward transmission from 13 passenger cases resulted in a total of 59 cases in six of eight HSE health regions in Ireland, necessitating national oversight of the outbreak. We calculated high attack rates, ranging plausibly from 9.8 % to 17.8% despite low flight occupancy and lack of passenger proximity onboard. On the flight date, 14-day Covid-19 case incidence was lower than 5 per 100,000 in Ireland, compared with 190 at time of writing (14 October 2020), permitting close focus on tracing countrywide."
The authors said rapid contact tracing can limit onward spread and recommended face coverings on-board.
"Restriction of movement on arrival and robust contact tracing are essential to limit propagation post-flight," it said.
The government is already considering a five-day quarantine period for passengers entering from overseas and on Saturday Taoiseach Micheál Martin said other quarantining measures were being considered.