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Factfind: Do we know the rate of travel transmission?

Tánaiste Leo Varadkar said on Wednesday night that international travel accounts for fewer than 1% of Covid-19 cases

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AS ENHANCED RESTRICTIONS for people travelling into Ireland come into force, Tánaiste Leo Varadkar told a Fine Gael parliamentary party meeting on Wednesday night that international travel accounts for fewer than 1% of Covid-19 cases in Ireland at present.  

Separately, Taoiseach Micheál Martin told a Fianna Fáil parliamentary party meeting that of 800 people who travelled into Ireland by air on Tuesday, 397 of them were from holiday destinations

Since Christmas, there have been increasing concerns about new variants of Covid-19 from the UK, South Africa and Brazil, prompting the Government this week to bring in some measures to mitigate against the risk of importation. 

So, is it accurate to say that fewer than 1% of Covid-19 cases are due to international travel? Let’s look at that figure the Tánaiste used on Wednesday night and see where this data comes from.

HPSC

Information on transmission rates and Covid-19 outbreaks are included in weekly reports from the Health Protection Surveillance Centre (HPSC).

The figure for international travel is contained in its most recent 14-day report

According to HPSC data, between 13 and 26 January, 32,103 cases of Covid-19 were reported in Ireland. 

The “most likely source of transmission” in 143 of these cases – or 0.45% – was travel, which, as Varadkar said, is lower than 1% of cases.  

Screenshot 2021-01-28 at 11.07.22 - Display 2 Source: HPSC

The HPSC says that travel-related cases include people who acquire Covid-19 infection outside of Ireland – imported cases – as well as people who are infected directly by these imported cases. 

However, taking into account the reality of Covid-19′s spread and Ireland’s track and trace capabilities, there is a nuance behind this data. 

A person who travels into Ireland and later tests positive will be defined as a travel-related case based on information provided to Public Health teams. 

So, too, will any person they directly infect that Public Health teams or contact tracers are able to link to the original case. 

However, onward transmission – should it occur – will not be labelled ‘travel-related’.

A member of the National Public Health Emergency Team [NPHET] explained that cases linked to travel are generally low because it is hard to get exact figures. “It is quite difficult to accurately represent,” they said. 

That is because a person could be infected by a travel-related case but never know it. 

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Cases by “mostly likely transmission source” are set out under six categories by the HPSC – travel-related, close contact, healthcare setting [patient], healthcare setting [staff], community transmission, and under investigation. 

Therefore, if a person is infected by a travel-related case – let’s say hypothetically a taxi driver at Dublin Airport – they may not display symptoms for several days and so may not be able to pinpoint how they contracted Covid-19. 

In that case, their infection would most likely fall under community transmission, where the source of infection is unknown. That accounts for 5,404 cases between 13 and 26 January. 

The travel data alone doesn’t give you an accurate enough picture of [travel-related] cases,” the NPHET member said. 

Based on limited HPSC data, it is accurate to say that less than 1% of cases in Ireland between 13 and 26 January were travel-related. 

However, it does not consider the possibility of onward transmission of these cases which ultimately may not be detected or linked to a travel-related case. 

So the Tánaiste was right to use a figure that was publicly available, but it is impossible to accurately say if this figure is correct or not, and it is, in fact, likely to account for more cases. 

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