COVID rebound after Pfizer treatment likely due to robust immune response, study finds

By Leroy Leo and Julie Steenhuysen

(Reuters) – A recurrence of COVID-19 symptoms in some patients after taking Pfizer’s antiviral Paxlovid may be related to a robust rather than weak immune response, US government researchers reported on Thursday.

They concluded that prolonged use of the drug — beyond the recommended five days — was not necessary to reduce the risk of symptom recurrence, as some suggest, based on an intensive study of rebound in eight patients at the National Institutes of Health Clinical Center.

All of the patients in the study had developed robust immune responses, but the researchers found higher levels of antibodies in the patients who experienced a rebound.

The team said their data contradict the hypothesis that impaired immune responses are the reason for some patients’ symptom recurrence.

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“Our results suggest that these clinical rebounds are characterized by a more robust immune response rather than uncontrolled viral replication,” the team wrote.

The study, published in the journal Clinical Infectious Diseases, followed numerous reports from people who took Paxlovid as recommended within five days of infection and saw a return of symptoms after completing the five-day treatment.

President Joe Biden and the director of the National Institute of Allergy and Infectious Diseases, Dr. Anthony Fauci, both experienced a COVID rebound after taking the drug.

The cases raised concerns that antiviral treatment with two of Pfizer’s drugs could interfere with the development of a long-lasting immune response.

The study involved six people whose COVID symptoms returned after taking Paxlovid and two with rebound symptoms after apparent recovery who did not take the pills. Their answers were compared to a group of six people who had COVID but did not experience a rebound. All volunteers had been vaccinated and boosted, and all were infected with some version of the Omicron variant of the virus.

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Blood from study volunteers was extensively sampled to assess their immune responses during the acute infection phase and the rebound phase.

All rebound patients had experienced a significant improvement in their symptoms prior to their rebound. Of those who had a rebound after Paxlovid, four had milder symptoms than during their initial infection, one had the same severity, and one reported worse symptoms.

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None of the rebound patients required additional treatment or hospitalization.

Rebound symptoms may be caused in part by a robust immune response to residual virus in the airways, the study authors suggested. They concluded that the drug does not hamper the immune response in some individuals, as some had feared.

Larger and more detailed studies are needed to better understand the resurgence of COVID symptoms, the research team said, adding that the current data supports the need to isolate such patients.

The researchers also suggested that there is still a need to investigate longer courses of Paxlovid treatment in immunocompromised individuals where the immune response may be ineffective.

(Reporting by Leroy Leo in Bengaluru and Julie Steenhuysen in Chicago; Editing by Bill Berkrot)

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