Evidence of waning immunity in all Covid-19 vaccines after six months, Hiqa says

Nursing home studies suggest vaccinated residents ‘may have a lower baseline level’ of protection than general population

There is evidence of waning immunity in all Covid-19 vaccines six months after the final dose, a review of international research by the Health Information and Quality Authority (Hiqa) has found.

The report says Covid-19 vaccines provide protection “for at least six months post-vaccination”, with some research pointing to “potential waning of vaccine effectiveness for severe disease in individuals at higher risk”.

The healthcare regulator said any drop in vaccine efficacy among more vulnerable groups “would be a concern”. There was also “uncertainty” about the impacts of new variants of concern, it added.

“While some differences were observed between the various vaccines, there was evidence of waning effectiveness across almost all observational studies in the general population which examined effectiveness up to six months after the final regimen dose,” Hiqa said.


The health watchdog said one study from Public Health England suggested “waning vaccine effectiveness” against the dominant Delta variant.

Hiqa reviewed more than 50 studies on vaccine effectiveness and immunity, and provided advice on its findings to the National Immunisation Advisory Committee (Niac), which make recommendations to the Government on vaccines.

Studies showed Pfizer, Moderna and AstraZeneca vaccines were highly effective for fighting severe disease. Research looking at the one-shot Janssen vaccine found its effectiveness in preventing severe disease over time varied from 60 per cent to 91 per cent.

Hiqa said one study pointed to the Pfizer vaccine’s effectiveness having “a possible decline over time,” but noted vaccine efficacy exceeded 83 per cent “for symptomatic disease up to six months after vaccination”.


For people aged 65 and older, Hiqa said one study showed the effectiveness of the AstraZeneca vaccine dropped to as low as 36 per cent after six months.

Other studies pointed to rates of effectiveness for the Janssen vaccine of 68 percent for those over 60, and 64 per cent in over 70s who received Moderna.

Regarding immunocompromised people, two studies found the vaccines were “less effective” in preventing hospitalisation with the virus.

The regulator said studies of vaccinated residents in nursing homes found lower levels of immunity, which it said suggests residents “may have a lower baseline level” of vaccine immunity, compared to the general population.

Hiqa also looked at 16 studies into vaccine effectiveness for healthcare workers, which found similar rates of immunity to other studies of the general population.

It said the evidence showed vaccines provided “a high level of protection against severe disease, symptomatic infection and any infection”. It noted healthcare workers may still contract the virus, “particularly when combined with high incidence rates in the community”. This meant a continued risk of health services facing staffing pressures, or cases where the virus was transmitted from staff to vulnerable patients.

There was “limited and inconsistent evidence” from research on the efficacy and effectiveness of Covid-19 vaccines among people with underlying conditions. It was “unclear” if the protection of vaccines waned quicker in those with underlying conditions than the general population, Hiqa said.

Dr Mark O’Loughlin, Hiqa public health fellow, said the findings of the various studies should encourage anyone offered a booster vaccine to avail of it.

Jack Power

Jack Power

Jack Power is acting Europe Correspondent of The Irish Times