Three Irish children who had measles infected two American infants on a transatlantic flight, sparking an international passenger alert.
The incident, detailed in a new report by the country's disease watchdog, led to over 300 passengers being traced earlier this year.
Public health experts in Ireland, Europe and the United States had to contact passengers and found that two American children described as "babes in arms" later developed measles.
The infection was passed on although the American children were seated nine and 11 rows away from the infectious Irish youngsters .
The presence of the children, whose parents were unaware they had measles at the time, posed a "significant public health threat" to fellow passengers who were not immune to the disease. The plane which travelled back and forth from Ireland to the US had 321 passengers who were potentially exposed to measles, according to the Health Protection Surveillance Centre (HPSC).
The three children from the one family, who had not received the MMR vaccine, were suffering sniffles and coughs which their parents believed were due to a cold.
They developed a rash a day after arriving in the US and travelled back home a few days later. It transpired another child in the family had been diagnosed with measles eight days before their departure but their GP failed to report the case to public health authorities. If the GP had done so, air travel would not have been advised for the three youngsters as they would be considered a risk for the disease. Irish authorities were alerted about the incident when the Irish children had travelled back home and were diagnosed with measles.
Public health authorities in the US then tracked their passengers while others were traced in Ireland, the UK and Spain.
The report from the HPSC noted that although there was considerable distance between the two sets of children it is possible they were in close proximity at one stage.
However, it pointed out that, even without this proximity, measles transmission through aerosolisation has been previously reported.
Cases have been reported of where people became infected after attending the same GP surgery hours after a diagnosed patient left.
The incident happened at a time when there was a rise in the outbreak of measles in Ireland -- it had the second highest incidence rate in Europe in the first three months of the year.
The report said measles is one of the most infectious disease known to man and potentially fatal.
The report highlights the infectiousness of measles and the importance of vaccination in preventing disease. Parents "should be informed and made aware of the implications to their children and to other children and older individuals when they do not vaccinate their children with MMR".