Coronavirus: Herd immunity may take multiple waves of infection – study

Ireland and other European countries are at risk of multiple further waves of infection from Covid-19 until the population develops possible herd immunity, according to new research led by an Irish scientist.

Although the current outbreak of the disease is peaking about now, there could be at least three further outbreaks in Ireland after the current one ends, the research by a team at King’s College London suggests.

Less than 20 per cent of the population of Ireland and six other countries would have possible immunity after the first wave of the infection, meaning the rest of the population could be open to infection in further waves, it said.

In Ireland, at most 16.7 per cent of the population would be affected, compared with 19.6 per cent in the UK, the researchers estimated.

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In the absence of a vaccine, this could mean at least three further waves of the virus before possible herd immunity would be acquired, according to lead author Dr Rosalyn Moran.

However, the number of subsequent waves could increase to nine under a different forecasting model used in the study that assumed fewer people were infected in the first wave due to more effective social distancing.

Herd immunity, the proportion of the population that needs to be immune to a disease to interrupt its transmission, is assumed at 60 per cent in the study.

“The notion that the first wave of infection is close to its peak begs the question of whether future peaks or ‘second waves’ are likely,” according to Dr Moran. “We sought to determine the current size of susceptible population for seven European countries, to estimate immunity levels following this first wave.

“We compare these numbers to the total population sizes of these countries, in order to investigate the potential for future peaks.”

The issue of herd immunity has been the subject of intense debate however during the Covid-19 pandemic with WHO epidemiologists warning in recent days that there was no proof yet that having had a coronavirus infection will confer immunity for a significant period of time.

According to the study, a large proportion of the total population in each country will remain without immunity after the current wave of infection. “This suggests that in the absence of strong seasonal effects, new medications or more comprehensive contact tracing, a further set of epidemic waves in different geographic centres are likely. These findings may have implications for ‘exit strategies’ from any lockdown stage.”

First peak

Ireland’s current outbreak is predicted to peak next Wednesday under one model used in the study, and to have peaked on April 9th under another. The estimated number of daily cases varies between 392 and 720 under the two models, while the cumulative death toll is put at 1,008-1,250.

In the UK, the two models suggest about 50,000 deaths in a first wave of infection.

“Both models predicted that we are currently nearing or past the peak of daily case rates in all seven countries,” says Dr Moran.

“However, the estimates suggest that after this cycle, more than 80 per cent of each country’s total population in all countries studied remain susceptible. Therefore, we assume that future cycles will occur.”

The authors say there is a trade-off between the size of the susceptible population and the length of a cycle of infection, and they say it is possible specific populations within an outbreak area could reach potential herd immunity after the current cycle.

The research has not yet been peer reviewed and the authors acknowledge it was conducted “with haste” due to the current crisis.

In Ireland, modelling research conducted for the National Public Health Emergency Team has found the current outbreak is plateauing about now, but has also warned of possible future surges after the easing of restrictions if adherence to social distancing and other measures falls.