New York, Nov 30 (IANS): Covid-19 infections in the 'fully' vaccinated are rare but are more common and severe in people with weaker immune systems, according to a real-world retrospective study led by the US drug maker Pfizer involving nearly 1.2 million people.

The immunocompromised people include those with advanced HIV/AIDS, cancer, kidney disease, rheumatologic or other inflammatory conditions, other immune conditions, and bone marrow or organ transplant recipients.

The findings, published in the Journal of Medical Economics, showed that the proportion of people with breakthrough infections was three times higher among immunocompromised individuals (0.18 per cent) than among the reference group of non-immunocompromised individuals (0.06 per cent).

Among the fully vaccinated people only 0.08 per cent experienced a breakthrough infection. However, although immunocompromised individuals represented just 18 per cent of those studied, they accounted for over 38 per cent of infections; nearly 60 per cent of all hospitalisations; and 100 per cent of deaths.

"The results supplement other real-world studies, and support the introduction of a third dose of a Covid-19 vaccine to increase protection among the immunocompromised individuals," said Manuela Di Fusco, lead author from the Pfizer Health Economics and Outcomes Research team.

"Several countries are currently experiencing a resurgence of SARS-CoV-2 infections despite the rollout of mass vaccination programmes. While Covid-19 mRNA vaccines help protect people from getting infected and severely ill, the risk of breakthrough infections in fully vaccinated people is not completely eliminated," Di Fusco added.

Researchers at Pfizer analysed healthcare records of 1,277,747 people aged 16 or over who had received two doses of the BNT162b2 Covid-19 mRNA vaccine between December 10, 2020 and July 8, 2021. Of this group, 225,796 were identified as immunocompromised.

The study results showed that a total of 978 breakthrough infections occurred among 1,176,907 fully vaccinated individuals who had at least two weeks of follow-up after the second dose. Of these 124 breakthrough infections required hospital treatment -- 74 were immunocompromised, and two people died (both immunocompromised). About 2 per cent of all breakthrough infections occurred in those who were immunocompromised.

Several countries, including the US and the UK, Israel, France, have authorised an additional dose of an approved Covid-19 mRNA vaccine in certain immunocompromised individuals.

The World Health Organisation (WHO), in October, recommended that people with weakened immune systems should receive an additional dose of a Covid-19 vaccine, due to their higher risk of breakthrough infections after standard vaccination.