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Clinical Context

The application of the messenger RNA (mRNA)-based COVID-19 vaccines has been one of the most fundamental efforts to fight the pandemic in the United States, and a recent study by Thompson and colleagues suggests that these vaccines remain highly efficacious, even in the face of the Delta and Omicron variants. Researchers used data from the VISION network, similar to the current study, to assess vaccine efficacy during surges related to Delta and Omicron. The results of this research were published in the January 28, 2022 issue of Morbidity and Mortality Weekly Report.^[1]

Nearly a quarter of a million (241,204) emergency department (ED) or urgent care (UC) encounters for acute respiratory illness were available for analysis, as were 93,408 hospitalizations for respiratory illness. The efficacy of the mRNA vaccine against confirmed infection with the Delta variant in the ED/UC setting was 86% within 6 months after dose 2, but efficacy increased to 94% after 3 doses. The respective vaccine efficacy results during the Omicron surge were 52% and 82%.

Having 2 vaccine doses in the past 6 months was 90% effective against laboratory-confirmed COVID-19--related hospitalization during the Delta surge, and the respective vaccine efficacy for 3 doses was 94%. During the Omicron surge, these respective values for vaccine efficacy were 81% and 90%.

There was a concerning trend toward reduced vaccine efficacy at 6 months or more since the second dose of COVID-19 vaccine. There is little information on weaning of immunity after the third dose of the mRNA vaccine, and the current study by Ferdinands and colleagues addresses this issue.

Study Synopsis and Perspective

Booster shots of the Pfizer/BioNTech and Moderna COVID-19 vaccines lost some effectiveness after 4 months but still did a good job of keeping people out of the hospital during the Omicron surge, a study shows.

During the time when the Omicron variant dominated, the vaccines provided 87% effectiveness against emergency room visits and 91% effectiveness against hospitalizations 2 months after the booster, the study showed. Four months after the booster shot, effectiveness dropped to 66% against ED visits and 78% against hospitalizations.

The study, published in the Morbidity and Mortality Weekly Report^[2], looked at 241,204 ED visits and 93,408 hospitalizations in 10 states from August 2021 to January 22, 2022. The Centers for Disease Control and Prevention (CDC), who published the report, said about 10% of the people were boosted and more than half the people hospitalized were older than age 65 years.

The study was no surprise because previous research showed vaccine and booster effectiveness wanes over time, but it appears the booster effectiveness against the Delta variant was stronger than against Omicron, the CDC said. The highly transmissible Omicron variant now accounts for almost 100% of COVID-19 cases in the United States.

The findings about the period when Omicron dominated were based on a small sample of fewer than 200 patients who had gotten the booster at least 4 months earlier.

Overall, the study provided more proof that vaccines work and keep people out of the hospital, said Michael Saag, an infectious disease doctor at the University of Alabama at Birmingham.

“Anecdotally, I’m seeing very few people die who got boosted,” he told Associated Press.^[3] “The vaccines are still working.”

In a separate report on Friday,^[4] the CDC changed its guidance on boosting for people with weakened immune systems. Those people should get boosted 3 months after completing the primary series of Pfizer/BioNTech or Moderna vaccines, not 5 months, the agency said.

About 7 million American adults are considered immunocompromised, Kaiser Health News has reported,^[5] including people who have certain medical conditions that impair their immune response or who take immune-suppressing drugs due to organ transplants, cancer, or autoimmune diseases.

The CDC recommended fourth shots for immunocompromised people in October.

The CDC also changed its guidance for immunocompromised people who received the one-dose Johnson & Johnson vaccine, saying they should get a second dose after 28 days, then get a booster of one of the mRNA vaccines.

Further, the CDC said people do not need to delay COVID-19 vaccination after receiving monoclonal antibodies or convalescent plasma.

Clinical Implications

• A previous study by Thompson and colleagues found that the mRNA COVID-19 vaccines were effective in recent surges, with greater efficacy against severe illness compared with more mild infection and greater efficacy against the Delta vs Omicron variants. Three doses of the COVID-19 vaccine offered increased protection vs 2 doses.
• The current study by Ferdinands and colleagues demonstrates that mRNA COVID-19 vaccine efficacy was lower overall during the Omicron vs Delta surge periods. The efficacy of the third dose of COVID-19 vaccine was lower at ≥ 4 months after application, and this decrement in efficacy appeared most dramatic in the prevention of ED/CU encounters during the Omicron surge.
• Implications for the healthcare team: New data confirm the value of a third dose of mRNA COVID-19 vaccine, but there is evidence that the protection associated with the third dose wanes over months. A new schedule of vaccination against COVID-19 may be forthcoming.

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