TORONTO -- A study involving more than a million fully vaccinated people found that COVID-19 breakthrough infections were more common among immunocompromised individuals and were far more likely to lead to hospitalization or death.

Researchers from Pfizer published their findings in the Journal of Medical Economics on Tuesday. They looked at 1.2 million people in the U.S. who had received both doses of the company's mRNA vaccine for COVID-19 between December 2020 and July 2021 in what they claim is the largest study of its kind.

“Several countries are currently experiencing a resurgence of SARS-CoV-2 infections despite the rollout of mass vaccination programs. 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,” lead author Manuela Di Fusco said in a news release.

Of the cohort, 17.7 per cent were identified as immunocompromised. Some of the immunocompromising conditions in the cohort included solid tumours, kidney disease, inflammatory diseases, organ transplants and HIV/AIDS.

However, immunocompromised participants accounted for 38.2 per cent of all breakthrough infections, 59.7 per cent of all hospitalizations and 100 per cent of all deaths.

Proportionally, the risk of breakthrough infections was three times higher among immunocompromised individuals. These individuals had a breakthrough infection rate of 0.18 per cent, compared to 0.06 per cent among non-immunocompromised individuals.

"While some COVID-19 vaccine breakthrough infections among those who are fully vaccinated against COVID-19 are expected, the findings of this study show that they are rare and less likely to result in hospitalization or death in those without an (immunocompromising) condition," the authors wrote.

Among the immunocompromised participants, organ transplant recipients were found to have the shortest time to infection as well as the highest rates of breakthrough infections.

The researchers say their findings underscore the need for health authorities to offer third doses for immunocompromised individuals, especially as vaccine immunity wanes and new variants such as Omicron emerge.

Health Canada gave the green light to use the Pfizer-BioNTech COVID-19 vaccine as a booster shot for adults. The rollout of these third doses has varied among the provinces and territories, with most prioritizing seniors, immunocompromised individuals, health-care workers, First Nations communities and those who received a non-mRNA vaccine.

“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," Di Fusco said.