Vaccines provide long-term care residents with substantial protection against severe outcomes from COVID-19. But these vulnerable adults may also face greater odds of adverse events after repeat doses, a new study of third-dose recipients finds.

Study participants included 280 residents from six long-term care facilities in Quebec, Canada. Residents received 100 micrograms of a Pfizer or Moderna mRNA booster dose (third shot) between Oct. 18 and Nov. 1, 2021. 

Investigators looked for a variety of systemic adverse events (SAEs), including fever, gastrointestinal symptoms, tachycardia, desaturation, respiratory distress, hypotension and hospital transfer during the first 48 hours following a shot. They also collected data on COVID-19 infection dates and vaccination series combinations.

Prior infection ups risk

In comparison with earlier reports of adverse events tied to third doses, the findings indicated a high proportion of systemic adverse events, specifically among LTC residents who had weathered a prior infection, they reported. 

Residents had varying rates of SAEs depending on the combination of vaccines. Those who received a Moderna-Pfizer-Moderna series had the highest rates, with SAEs of 46% (of which 40% were fever). This was compared with 16% for Moderna-Pfizer-Moderna, 6% for M-M-P and 2% for P-P-P.

These findings suggest that the Moderna 100-microgram booster dose and mix-and-match vaccination may have increased inflammatory responses, the researchers theorized. The 100-microgram Moderna dosage was planned before the FDA approved a lowered booster dose of 50 micrograms for that company’s shot, they noted.

Mitigating residents’ risk

Studies continue to show the high levels of protection that vaccines provide older adults against hospitalization and death from COVID-19. But the Quebec study results underscore the need to anticipate and mitigate risk when planning vaccinations in the long-term care population, the authors wrote.

“As additional booster doses are considered due to waning immunity and variants, examination of past vaccines and intervals, immunity status and booster dosage may be required to weigh their potential benefits against the risk of adverse effects,” they concluded.

In related news, study results released this week by the Centers for Disease Control and Prevention have found that seniors receive 80% protection against severe omicron illness with a fourth vaccine dose. And a study from Israel, also published this week, found that the third and fourth Pfizer doses in adults aged 60 years or older were associated with a significant increase in immune response at two weeks after the vaccination, and with no major adverse events.

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