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How Long Do COVID Boosters Remain Effective?

  • Authors: News Author: Ralph Ellis; CME Author: Charles P. Vega, MD
  • CME / ABIM MOC / CE Released: 4/15/2022
  • Valid for credit through: 4/15/2023
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  • Credits Available

    Physicians - maximum of 0.25 AMA PRA Category 1 Credit(s)™

    ABIM Diplomates - maximum of 0.25 ABIM MOC points

    Nurses - 0.25 ANCC Contact Hour(s) (0.25 contact hours are in the area of pharmacology)

    Pharmacists - 0.25 Knowledge-based ACPE (0.025 CEUs)

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Target Audience and Goal Statement

This activity is intended for primary care physicians, infectious disease specialists, nurses, pharmacists, physician assistants, and other members of the healthcare team who care for patients at risk for COVID-19.

The goal of this activity is that learners will be better able to discern the efficacy of messenger RNA (mRNA) vaccination against COVID-19 over time.

Upon completion of this activity, participants will:

  • Analyze the efficacy of mRNA vaccines against the Delta and Omicron variants of COVID-19
  • Assess the stability of protection associated with the third dose of COVID-19 vaccines
  • Outline implications for the healthcare team


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News Author

  • Ralph Ellis

    Freelance writer, Medscape


    Disclosure: Ralph Ellis has disclosed no relevant financial relationships.

CME Author

  • Charles P. Vega, MD

    Health Sciences Clinical Professor of Family Medicine
    University of California, Irvine School of Medicine


    Disclosure: Charles P. Vega, MD, has disclosed the following relevant financial relationships:
    Served as an advisor or consultant for: GlaxoSmithKline; Johnson & Johnson Pharmaceutical Research & Development, L.L.C.

Editor/Nurse Planner

  • Leigh A. Schmidt, MSN, RN, CMSRN, CNE, CHCP

    Associate Director, Accreditation and Compliance
    Medscape, LLC


    Disclosure: Leigh A. Schmidt, MSN, RN, CMSRN, CNE, CHCP, has disclosed no relevant financial relationships.

Compliance Reviewers

  • Esther Nyarko, PharmD

    Director, Accreditation and Compliance
    Medscape, LLC


    Disclosure: Esther Nyarko, PharmD, has disclosed no relevant financial relationships.

  • Yaisanet Oyola, MD

    Associate Director, Accreditation and Compliance
    Medscape, LLC


    Disclosure: Yaisanet Oyola, MD, has no relevant financial relationships.

PA Planner

  • Jennifer Hakkarainen, PA-C

    Medical Education Director, Medscape, LLC


    Disclosure: Jennifer Hakkarainen, PA-C, has disclosed no relevant financial relationships.

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This activity has been peer reviewed and the reviewer has no relevant financial relationships.

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How Long Do COVID Boosters Remain Effective?

Authors: News Author: Ralph Ellis; CME Author: Charles P. Vega, MDFaculty and Disclosures

CME / ABIM MOC / CE Released: 4/15/2022

Valid for credit through: 4/15/2023


Note: The information on the coronavirus outbreak is continually evolving. The content within this activity serves as a historical reference to the information that was available at the time of this publication. We continue to add to the collection of activities on this subject as new information becomes available. It is the policy of Medscape Education to avoid the mention of brand names or specific manufacturers in accredited educational activities. However, manufacturer names related to COVID-19 vaccines may be provided in this activity to promote clarity. The use of manufacturer names should not be viewed as an endorsement by Medscape of any specific product or manufacturer.

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.

Study Highlights

  • Researchers drew study data from the VISION Network, which includes 8 major health systems across the United States. Health encounters eligible for assessment featured adults aged ≥ 18 years presenting with symptoms of COVID-19. All patients had molecular testing for SARS-CoV-2.
  • The study period began on August 26, 2021, which was 14 days after the first US recommendation for a third dose of the mRNA COVID-19 vaccine.
  • The study excluded patients who had received just 1 dose of the vaccine as well as persons who received any vaccine dose within 14 days of illness.
  • Researchers calculated vaccine efficacy as the rate of positive testing between vaccinated and unvaccinated patients. This result was adjusted to account for epidemiologic and personal health variables.
  • 241,204 ED/UC encounters were available for analysis, 77% of which occurred during the Delta surge and 23% of which occurred during the Omicron surge.
  • In all, 46% of patients seeking care in ED/UC were unvaccinated, and 44% had received 2 vaccine doses. Just 10% had received 3 vaccine doses.
  • During the Delta surge, the vaccine efficacy for the 3-dose series was 97% in ED/UC visits when the final dose was delivered within 2 months of illness. The respective vaccine efficacy when the third dose was delivered ≥ 4 months or more before illness fell to 89%.
  • Vaccine efficacy against ED/UC encounters during the Omicron surge was just 37% at ≥ 5 months after a 2-dose series. The respective vaccine efficacy increased to 87% with the third dose within 2 months of the onset of illness; however, vaccine efficacy waned to 66% when the third dose was provided 4 to 5 months earlier, and it fell further to 31% among persons who received the third dose > 5 months before illness. The authors warned that this last result is imprecise because of a low sample size.
  • Data for 93,408 hospitalizations was available, 89% of which occurred during the Delta surge.
  • In all, 43% of patients hospitalized were unvaccinated, and 45% had received 2 vaccine doses. Just 12% had received 3 vaccine doses.
  • Vaccine efficacy against hospitalization during the Delta surge was 96% with the third vaccine dose within 2 months of illness, and it declined to 76% at 4 months or more after the third dose.
  • Vaccine efficacy against hospitalization during the Omicron surge was 91% with the third vaccine dose within 2 months of illness, and it declined to 78% at 4 months or more after the third dose.

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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