Delta variant could do ‘significant damage’ in partially vaccinated population, Nolan says

Prof Philip Nolan outlines modelling behind advice that led to postponed reopening

The general resumption of indoor services in pubs and restaurants that had originally been planned for next week would have risked significant surges in cases of Covid-19, with the Delta variant leading to "very significant damage" if allowed to spread in a partially vaccinated population, Prof Philip Nolan has said.

Prof Nolan, chair of the National Public Health Emergency Team (Nphet) epidemiological modelling advisory group, has outlined the modelling used by the group to predict there could be as many as 681,900 new Covid-19 cases and 2,170 further deaths from the disease by the end of September in a pessimistic scenario.

In response to the advice, the Government agreed to halt the original plan and that pubs and restaurants should not reopen indoors until a workable system is in place to prove customers have been fully vaccinated.

“It has been a difficult and disappointing week for many, as the rise to dominance of the delta variant has delayed plans for wider reopening,” wrote Prof Nolan on Twitter on Wednesday. “But the likely impact of delta is stark, and caution is well advised.


“The model has been adapted to include the effect of vaccination, where we can vary assumptions on the effectiveness of vaccination in preventing asymptomatic infection, symptomatic infection, severe disease, and onward transmission.

“The starting point for a set of projections is to estimate the likely level of close social contact. We have a good estimate of what might happen this summer, based on our experience of last summer.

“The summer 2020 reopening led to [the] reproduction number increasing from below one to somewhere between 1.4 and 1.6. This began in late June 2020, and the level of viral transmission was more or less constant at this level through July and August 2020.

“However, because the level of infection in late June 2020 was very low, it was weeks before the underlying exponential growth became apparent. Essentially the seeds of the October 2020 surge were sown in July and August 2020,” he tweeted.

Reproduction number

The reproduction, or R, number is the number of people that one infected person will pass on a virus to, on average. If the value of the number is greater than one, the number of people getting infected is increasing.

“This is our starting assumption,” continued Prof Nolan. “The level of social mixing with summer reopening could either be moderate (the level which last year gave us an R of 1.4) or high (the level which last year gave us an R of 1.6).

“However, we are now dealing with a different variant, which is much more transmissible, and so the same levels of social mixing will lead to much higher levels of transmission and much faster growth in cases, which will be attenuated by progressive vaccination.

“The vaccination input to the model includes the impact of [the] delta variant on vaccine effectiveness: a modest reduction in overall vaccine effectiveness, and a more marked and transient reduction in vaccine effectiveness in the interval between first and second doses.

“A counterfactual scenario, where we model with the alpha variant only, shows that without delta we could have opened on July 5th with little increase in cases or admissions.

“However, even the most optimistic scenario with the delta variant shows opening on July 5th risks a significant surge in cases; note that case numbers rise very slowly through July and then surge in August and September, this being the nature of exponential growth.”


Prof Nolan said a rise in new cases will “inevitably lead to hospitalisation and mortality”. Though the rates in this regard will be far less than we experienced without vaccination, there was nonetheless the risk of “a long wave of disease” leading to “a significant number of adverse outcomes”.

“While 70-80 per cent of cases will be in people under 40, there will be a lot of infections and a lot of adverse outcomes in people over 40,” he said. “About 70 per cent of the hospitalisations and over 99 per cent of the deaths would be in people over 40.

“Why would this happen if everyone is vaccinated? Vaccines offer extraordinary protection, but not perfect. We have almost 500,000 people aged 70 and over. Even if the vaccine is 95 per cent effective in preventing severe disease, 25,000 people remain vulnerable.

“All this illustrates one thing: a variant with a transmission advantage can do very significant damage if we let it spread in a partially vaccinated population, the scale of the damage depends on the transmission advantage, and it starts slowly and escalates rapidly.”

Colin Gleeson

Colin Gleeson

Colin Gleeson is an Irish Times reporter