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The COVID-19 antiviral Paxlovid (nirmatrelvir/ritonavir) offers 96% protection against hospitalization in high-risk patients who are incompletely vaccinated, a new omicron-era study has found. It is also significantly protective in older adults and immunosuppressed patients under certain conditions, researchers said.

The investigation included data from 8,402 outpatients treated in Quebec province, Canada, between March 15 and Oct. 15, 2022. Researchers analyzed the risk of COVID-19-associated hospitalization occurring within 30 days of followup, compared with risk among untreated controls.

Regardless of vaccination status, Paxlovid treatment was associated with a 69% reduced relative risk of hospitalization, they found. And the benefits were even more pronounced (96%) in patients who had not completed a full primary COVID-19 vaccination course. 

Older adults and vaccine timing

In contrast, there was no protection against hospitalization for patients at high risk of severe disease who completed their primary vaccination series of shots less than six months ago. The latter finding suggests that a booster dose at six months “remains an effective method for preventing severe illness,” the researchers wrote.

But when patients who had a full primary series of vaccinations were sorted into groups, two subgroups stood out. High-risk patients aged 70 years and older were significantly protected when their last vaccine dose was received at least six months beforehand.

In addition, severely immunocompromised patients also received significant protection, no matter their vaccination status, the researchers reported.

Omicron surge

Notably, the study took place during the omicron variants BA.2 and BA.4/5 surge. The results “are consistent with previous studies that demonstrated nirmatrelvir/ritonavir treatment and COVID-19 vaccination improve clinical outcomes of SARS-CoV-2 infected patients,” the researchers concluded.

Full findings were published in Clinical Infectious Diseases.

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