Covid-19: Vaccine defence fading but other factors influence risk

Delta variant not proving to be bigger foe to vaccinated but booster could be tweaked

The protection afforded by Covid-19 vaccines is waning, studies clearly show, but this does not mean vaccinated people are at greater risk of serious illness.

This is because the vaccines were designed to provide protection against serious outcomes such as hospitalisation and death, and they continue to provide this.

The most recent UK data shows protection against infection provided by the Pfizer vaccine dropping from 88 per cent one month after vaccination to 74 per cent at five to six months. Protection from the AstraZeneca vaccine dropped from 77 per cent to 67 per cent.

Data from Israel and other countries also points to a drop in vaccine effectiveness, prompting calls for booster shots in the autumn.


So far, however, according to most studies, the protection provided against hospitalisation and death remains high – about 80 per cent.

Although the neutralising antibodies induced by the vaccine appear to decline over time, the body has other means of protection against viruses, such as memory B and T cells, and these levels appear to be maintained.

Behavioural elements

In Israel, protection against infection was maintained among older age-groups, but it fell in those aged between 18 and 49.

Scientists believe this points to behavioural factors at play.

The apparent drop in effectiveness could be related to the fact people are likely to be repeatedly exposed to the virus as time passes, increasing the opportunities for the virus to break through the immune system’s defences.

The details emerging from the many studies in different countries are not surprising. It was always known the vaccines would not be 100 per cent effective. Real-world effectiveness was always going to be a sterner test of performance than measures of vaccine efficacy in clinical trials. Antibodies always decline over time and it is harder to stop infection than symptomatic illness.

Up to 60 per cent of hospitalised patients in Israel are vaccinated, leading to questions about dose effectiveness. But any analysis has to take account of confounding factors such as high rates of vaccination and varying uptakes among different age groups.

Confidence rattled

"After accounting for the vaccination rates and stratifying by age groups, we can see that the vaccines retain high effectiveness (85-95 per cent) versus severe disease," Dr Jeffrey Morris, a biostatistician at the University of Pennsylvania, points out.

The highly transmissible Delta variant has rattled the early confidence about vaccines but it has not proven itself a more lethal foe. For now, it does seem as if the current crop of vaccines will manage to hold the virus in check, provided sufficient numbers of people get vaccinated.

A time will come when the general population needs boosters but, as virologist Dr Gerald Barry of UCD has pointed out, it would make sense for that booster to be tweaked to target the Delta variant, rather than being simply a repeat of what is currently available.

Paul Cullen

Paul Cullen

Paul Cullen is a former heath editor of The Irish Times.