COVID-19 infections in the fully vaccinated are rare but more common and severe in people with weaker immune systems, according to a Pfizer-led study involving nearly 1.2 million people.

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The research, published in the Journal of Medical Economics, found that within a pool of 1,277,747 fully vaccinated people in the US only 0.08 per cent experienced a breakthrough infection between December 10, 2020, and July 8, 2021. 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 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). "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, study lead author from the Pfizer Health Economics and Outcomes Research team. "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 said.

Identifying individuals in the US population who had received two doses of the Pfizer/BioNTech COVID-19 mRNA vaccine, the study looked to examine breakthrough infections among those with and without a weakened immune system. From a large collection of US healthcare administrative data, the team analysed healthcare records of 1,277,747 people aged 16 or over who had received two doses of the Pfizer COVID-19 mRNA vaccine.

Of this group, 225,796 (17.7 per cent) were identified as immunocompromised -- including people with advanced HIV/AIDS, cancer, kidney disease, rheumatologic or other inflammatory conditions, other immune conditions, and bone marrow or organ transplant recipients.

The study results showed a total of 978 (0.08 per cent) breakthrough infections occurred among 1,176,907 fully vaccinated individuals who had at least two weeks of follow-up after the second dose. As many as 124 (12.7 per cent) of these breakthrough infections required hospital treatment. Of them, 74 (59.7 per cent) were immunocompromised, and two people died (both immunocompromised), the researchers said.

Two per cent of all breakthrough infections occurred in those who were immunocompromised, they said. "While the study leveraged a large dataset that has been previously used for healthcare research in the US, the findings should be interpreted in the context of multiple limitations, including coding accuracy and data representativeness beyond the study period," Di Fusco added.