Coronavirus: Ireland has ‘no significant’ herd immunity, study shows

The vast majority of the Irish population are still vulnerable to catching a coronavirus infection, according to early results from a study to investigate exposure to Covid-19 in the population.

Initial findings from research into a random sample of healthy people in Dublin and Sligo indicate that less than 5 per of the population have been exposed to the disease.

They show the State is still some way off achieving herd immunity, where 60-90 per cent of population become immune to a virus to halt it spreading.

More than 5,000 letters were sent out in Dublin and Sligo – areas of the country with higher and lower known levels of infection respectively – to participate in blood tests for Covid-19 antibodies.

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The research, known as a seroprevalence study, measures antibodies in the blood of healthy people to better understand past rates of infection across the population. A person develops antibodies to protect against future infections from the illness.

Almost 2,500 people participated in the study with blood tests taken from final participants last week, one month after the study was launched.

Dr Cillian De Gascun, director of UCD's National Virus Reference Laboratory, which helped co-ordinate the study, said that the rate was slightly higher in Dublin than in Sligo. He said that finding out that less than 5 per cent of the population as a whole being exposed to the virus was "a little bit disappointing".

“What that means is there is a very large susceptible population there, and that is one of the reasons why we have to be so careful with moving through the reopening phases,” he said. “Because so many people have not had the infection and if the virus kicks off again, it will transmit very readily to people purely because there is no significant level of population immunity at this point.”

Follow-up study

Dr De Gascun said researchers would look to repeat the tests with different sections of the population over the next six to 12 months and return to people in whom antibodies were detected to see if they would participate in a follow-up study to assess how long they have immunity.

“We are seeing suggestions that antibodies probably don’t last much past three or four months but it is still early days, so if we can get a cohort of people together and resample them at intervals over the next six to 12 months it would be interesting to see how long the antibodies last,” he said.

He expressed disappointment at a response rate of less than 50 per cent out of the more than 5,000 people contacted. He thought more people would have participated given the profile of the virus.

Serological surveys are considered a valuable tool to measure the extent of the pandemic given the number of people who can have the virus but show no symptoms. The initial results of the Irish seroprevalence study is similar to analysis in other countries.

A study of 60,000 people in Spain – one of the countries worst affected by the Covid-19 pandemic – found that about 5 per cent of the population had developed antibodies, according to a report published in the Lancet.

The area around Madrid, the hardest-hit part of the country, had more than 10 per cent prevalence of antibodies, more than five times the level of less populated coastal provinces.

A study by Germany's infectious diseases agency, the Robert Koch Institute, detected antibodies in only 1.3 per cent of 12,000 blood donors tested earlier this month. More than half the residents tested in Italy's northern province of Bergamo – one of the worst affected areas in the world – were last month found to have Covid-19 antibodies.