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People with HIV have a higher rate of breakthrough Covid-19 infections after vaccination, compared to people without HIV, according to a study.

A "breakthrough infection" is when a fully vaccinated person contracts Covid.

The results, published in JAMA Network Open, showed that the chance of a positive SARS-CoV-2 test result or a Covid-19 diagnosis within nine months after full vaccination, though low, was 28 per cent higher among people with HIV.

The risk of breakthrough infection during the period examined was 3.8 per cent for the people without HIV and 4.4 per cent for the people with HIV. The rates of breakthrough are much lower than the rate of Covid in unvaccinated people, suggesting a strong protective effect of vaccination, said Keri Althoff, Associate Professor at the Johns Hopkins Bloomberg School of Public Health.

Public health officials have had concerns about potentially elevated Covid risk among people with weakened immune systems, including those with HIV, since the start of the pandemic.

The US Centres for Disease Control and Prevention currently recommends that people who are "moderately or severely immunocompromised" - a category that includes people with HIV who are untreated or have low CD4 T-cell counts (below 200 cells per microliter) - receive an extra dose of vaccine as part of their primary vaccination series, followed by a booster.

For their study, the researchers analysed anonymised health records among nearly 114,000 people fully vaccinated with either two doses of mRNA vaccines or one dose of the J&J viral vector vaccine as of June 30, 2021 through December 31, 2021.

Further, the study found an increasing risk of breakthrough with increasing immune suppression, measured via decreasing CD4 counts. Those with CD4 counts that signal moderate immune suppression in people with HIV - in the 200-350 cells/mm3 range - had a statistically significant increase in the risk of breakthrough compared to people without HIV.

That suggests, Althoff said, that people with HIV and moderate immune suppression may need to be included in the CDC's guidelines for additional doses of vaccine in the primary vaccination series.

"Policymakers who establish the guidelines should consider the benefits and risks of an additional dose of vaccine in the primary series not only for those with severe or untreated HIV, but also include those with moderate immune suppression or even all persons with HIV," said Sally Coburn, post-doctoral fellow in the Bloomberg School's Department of Epidemiology.