Nine-month post-Covid immunity ‘increases fully vaccinated numbers with single dose’

The extension in presumed period of immunity post-Covid-19 infection to nine months would increase the number of people considered fully vaccinated with a single dose, a new report says.

In advice to the State’s public health team, the State’s health service regulator, the Health Information and Quality Authority (Hiqa) said studies suggest most people develop “immune memory” after a Covid-19 infection that “lasts for at least nine months”.

Chief medical officer Dr Tony Holohan said on Wednesday Nphet had accepted Hiqa's advice on immunity and protection from reinfection to extend the presumed immunity period from six to nine months. The period may be extended further to 12 months later this year, he said.

Hiqa published research papers on Thursday morning showing the evidence behind its advice.

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Dr Máirín Ryan, Hiqa’s deputy chief executive and director of technology assessment, said increasing the period of presumed immunity has “widespread positive implications for people”.

“A change would also increase the number of under-50s who only need one dose of a Covid-19 vaccine to be considered fully vaccinated,” she said.

However, Hiqa warns that implementing a one-dose vaccine schedule for those previously infected was “problematic as it has been difficult to ascertain previous infection status”.

Scientists have argued that limiting people who have been previously infected by Covid-19 to a single dose of the vaccine could free up doses, speed up the rollout of the vaccination programme and deem more people to be fully vaccinated as the risk from more infectious variants looms.

Other benefits from the changed presumed immunity period include exemptions from serial testing for people who had Covid-19 in the past nine months.

Hiqa said it could also have implications for the rollout of the proposed “green certificates” that will allow vaccinated people to travel more freely internationally.

Studies showed the risk of reinfection from the coronavirus disease was “consistently low with no increase in infection risk over time,” said Dr Ryan

She said it was important that “any policy changes and the evidence behind them are clearly communicated and consistently applied”.

Hiqa sounds a note of uncertainty about new variants: “Our understanding of the impact of new variants on natural immunity is evolving rapidly and should be kept under review.”

The regulator’s advice to Nphet was based on a review of international evidence including 19 large cohort studies of reinfection involving more than 640,000 previously infected individuals.

The regulator’s paper was also informed by 13 studies on “immune memory response” along with expert opinion from the Covid-19 Expert Advisory Group.

NUI Maynooth professor of immunology Paul Moynagh has said the Government could make up any shortfall in vaccine supplies by first testing people for antibodies to see whether they have previously had Covid-19 and then administering a single dose to them if they had.

“The reality is that somebody who has been previously infected really only needs one dose of vaccine,” he said.

More than 250,000 people have been confirmed as being previously infected in the country but the actual number could three or four times that number, he said. He suggested a simple pin-prick of blood for an antibody test could determine those people who do not need a second dose.