Passengers on a flight to Dublin led to the spread of 59 Covid-19 cases around the country.
A report into the outbreak states the laboratory-confirmed cases were linked to an international flight into Ireland in summer 2020.
That 7.5 hour-long flight, in which 49 of its 283 seats were filled, is believed to have landed at Dublin Airport in June.
The HSE was unable to contact 11 of the 49 passengers on board, and one of the passengers refused to be tested for Covid-19.
Another of the passengers travelled away from Dublin and stayed in “shared accommodation” with 34 other people.
Of these, 25 - who were from all over the country - became infected.
The report by Eurosurveillance raises an issue about the effectiveness of the Passenger Locator Form tracking system that was operated by the Department of Justice at the time.
That system has only been in place electronically since August 26, from which date it is an offence for an air passenger not to have submitted their contact details in advance of flying into Ireland.
The report into the flight case states: “This outbreak demonstrates the potential for spread of SARS-CoV-2 linked to air travel.
“We calculated high attack rates, ranging plausibly from 9.8 % to 17.8% despite low flight occupancy and lack of passenger proximity onboard.”
And it adds: “Exposure possibilities for flight cases include in-flight, during overnight transfer/pre-flight or unknown acquisition before the flight.
“The incubation period for Covid-19 may be as short as two days, so the potential for in-flight/airport transmission exists in this outbreak.”
A national outbreak control team was convened with the involvement of the Departments of Public Health - including the Department of Public Health, HSE-South, Cork - to investigate, identify and interview cases, oversee contact tracing, and establish control measures.
Close contact passengers were defined as two seats in every direction from the first cases notified, according to the open-access infectious diseases medical journal Eurosurveillance.
They were initially traced and the cabin crew was risk-assessed to establish whether they would be classified as close contacts of the cases or whether they had any illness.
As transmission emerged, the journal’s October 21 report says, close contacts on the flight for the first eight presenting cases were identified and nine reachable contacts tested negative. With the emergence of further positives, the remaining passengers were offered testing where contactable.
Five further symptomatic positive cases were confirmed, some following medical assessment. An additional 15 passengers tested Covid-19 ‘not detected’.
The report states: “One passenger declined 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.
“As a result of the risk assessment, crew members were advised to quarantine for 14 days.”