Comparing how the Covid-19 outbreak is playing out in Ireland and the UK is a complex task, researchers have said after a Twitter thread by a former historian on the issue went viral.

Writer and researcher Dr Elaine Doyle penned a series of tweets comparing the situations in the UK and Ireland, noting that both countries had similar numbers of intensive care beds per 100,000 people before the crisis began.

But, she wrote, “as of Saturday 11 April, there have been 6.5 deaths per 100,000 people in Ireland. There have been 14.81 deaths per 100,000 people in the UK.” Doyle went on to suggest that the difference in the way the pandemic is progressing in the two countries is that Ireland took stronger action sooner.

“While Boris [Johnson] was telling the British people to wash their hands, our taoiseach was closing the schools. While Cheltenham was going ahead, and over 250,000 people were gathering in what would have been a massive super-spreader event, Ireland had cancelled St Patrick’s Day,” she wrote, adding that watching British media was “like living in bizarro-world” compared with the messages on Irish TV news.

In the UK, the government urged against socialising, mass gathering and non-essential travel from 16 March and went into full lockdown on 23 March.

“Technically, the UK went into lockdown *before* Ireland; but that’s not a fair comparison, as we were already operating our ‘delay phase’ from 12-27 March,” Doyle wrote.

She also noted that, at the time of her post, figures suggested Ireland had performed 8.69 tests per 1,000 people, while the UK had performed four tests per 1,000 people – with 269,598 people tested in total as of 11 April in the UK.

Dr William Hanage, an associate professor of epidemiology at Harvard University, said Doyle had made some good points, noting that he and many of his colleagues had been concerned about the UK’s early response to coronavirus, but cautioned that it was still too early to draw definitive conclusions.

“We are early on in the pandemic and it will remain to be seen how this will all pan out. However, it is unquestionable that major events such as the Cheltenham festival were major opportunities for super-spreading to occur,” he told the Guardian. “And the resulting surge can reasonably be expected to have been more severe than it would have been otherwise.”

But Prof Sheila Bird, a former programme leader at the MRC Biostatistics Unit, stressed there were many factors to consider when attempting to compare countries, including whether deaths were reported in the same way, and whether delays in reporting deaths had been taken into account.

“Third,” she said, “[is] urbanisation versus rurality of the respective populations – 83% urban for UK, 63% for Ireland.” In other words, a greater proportion of people in the UK live in towns or cities, which may contribute to the spread of a disease.

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Bird said it was also important to consider how well people adhered to social distancing and lockdowns in different countries, while age was also a factor: older people are known to be at greater risk of death from Covid-19. In the UK, Bird noted, 18% of the population was aged 65 or older, compared with only 13% in Ireland.

Paul Hunter, a professor in medicine at the University of East Anglia, said the UK might have had proportionately more cases, which would mean more deaths. While he accepted that the later implementation of social distancing measures in the UK might have contributed to the different outcomes, he said there were other factors to consider, including that the UK had a higher proportion of people from BAME communities – who have been found to be at higher risk from coronavirus – and that the UK was also more densely populated, had many areas of high poverty, and hospitals in London were stretched.

Keith Neal, an emeritus professor in epidemiology of infectious diseases at the University of Nottingham, added that the UK probably encountered its first infections earlier.

“The risk of introduction is related to the number of travellers coming back with an infection,” he said. “Although Ireland may have the same number of international travellers per head of population, your risk of first introductions is related to the number of travellers. The UK is 13 times larger in population than Ireland and London is much more of an international centre and hub than Dublin.”

That, said Neal, could be significant: “With doubling times of every two to three days, even a later introduction of the first infections by a week can have a very large effect.”

Professor Samuel McConkey, an infectious disease expert at RCSI University of Medicine and Health Sciences in Dublin, said it was premature to compare Ireland and the UK. “I’d reserve judgment on this for two or three years,” he said.

He said Ireland’s earlier adoption of restrictions, as well as London’s population density, may partly explain greater mortality rates in the UK. “We closed restaurants, pubs, creches, schools weeks before the UK. We had quite significant political cohesion. We had our national leader tell us, ‘folks this is really bad’.”

Seán L’Estrange, a University College Dublin sociologist who has compared international testing figures, challenged Ireland’s claim to be in the top tier. Ireland claimed to follow South Korea’s model of “test, track, trace, isolate” but was in fact “uncomfortably close” to countries most removed from the strategy, such as the UK, he said.

“There are very clear differences between the ROI and UK responses. They are not, however, as large or as pronounced as Irish authorities imagine and would like its public to believe.”