TORONTO — A study involving more than a million fully vaccinated people found that COVID-19 breakthrough infections were more common in immunocompromised individuals and more likely to be hospitalized or death.

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Researchers at Pfizer published their findings Journal of Medical Economics on Tuesday. They looked at 1.2 million people in the US who received both doses of the company’s mRNA vaccine for COVID-19 between December 2020 and July 2021, what it claims is the largest study of its kind.

“Many countries are currently experiencing a resurgence of SARS-CoV-2 infections despite the rollout of large-scale vaccination programmes. While COVID-19 mRNA vaccines help to protect people from becoming infected and seriously ill, the risk of breakthrough infection is not completely eliminated in those who are fully vaccinated,” said lead author Manuela Di Fusco said in a news release.

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Of the cohort, 17.7 percent identified as immunocompromised. Some of the immunological conditions in the cohort included solid tumors, kidney disease, inflammatory diseases, organ transplantation, and HIV/AIDS.

However, immunocompromised participants accounted for 38.2 percent of all successful infections, 59.7 percent of all hospitalizations, and 100 percent of all deaths.

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Proportionally, the risk of breakthrough infection was three times higher in immunocompromised individuals. These individuals had a success infection rate of 0.18 percent, compared to 0.06 percent in non-immunoprecipitated individuals.

“While some COVID-19 vaccine breakthrough infections are expected in people who are fully vaccinated against COVID-19, the findings of this study suggest that they are rare and require hospitalization in people without (immunocompromising) conditions.” less likely to occur or die.” wrote the authors.

Among immunocompromised participants, organ transplant recipients had the shortest time to infection as well as the highest rate of breakthrough infection.

The researchers say their findings underscore the need for health officials to offer a third dose for immunocompromised individuals, especially as immunity to the vaccine is reduced and newer versions such as Omicron emerge.

Health Canada gave the green light to use the Pfizer-BioNtech COVID-19 vaccine as a booster shot for adults. These third-dose rollouts have varied across provinces and territories, with the highest priority being senior citizens, immunocompromised individuals, health care workers, First Nations communities and non-mRNA vaccine recipients.

“The results complement other real-world studies and support the introduction of a third dose of a COVID-19 vaccine to increase protection among immunocompromised individuals,” Di Fusco said.