COVID-19 Breakthrough Infections

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A breakthrough infection is defined as becoming infected with COVID-19 after being fully vaccinated. Breakthrough cases are expected, as vaccines are not 100% effective. Breakthrough infections are typically, but not always, less serious than those occurring in unvaccinated individuals. 

The CDC is monitoring breakthrough infections via several different systems. Coronavirus Disease 2019 (COVID-19)-Associated Hospitalization Surveillance Network (COVID-NET) covers 10% of the United States population and is CDC’s source for important data on hospitalization rates associated with COVID-19. Hospitalization rates are updated weekly. COVID-NET also provides important clinical information on COVID-19-associated hospitalizations, including age group, sex, race/ethnicity and underlying health conditions. The National Healthcare Safety Network (NHSN) provides healthcare facilities, such as long-term care facilities (LTCFs), with a secure reporting platform for reporting outcomes and process measures in a systematic way. Additionally, health departments in 49 states are voluntarily reporting data on breakthrough cases. In evaluation of all available data, the CDC reports no unusual activity regarding any COVID variant or any particular patient group regarding breakthrough infections.

The available vaccines have been found to be highly effective against the Delta variant. A recent article published by the New England Journal of Medicine examined breakthrough infections in healthcare workers. Eighty-five percent of infections were found to be the alpha strain. Most patients with breakthrough cases had mild symptoms or were asymptomatic. Only about 19% of patients had persistent symptoms.

For further reading about vaccines currently available in the United States, please refer to our COVID-19 Vaccines and Boosters post on COVID Central.


Centers for Disease Control and Prevention

Bergwerk M, Gonen T, Lustig Y, Amit S, Lipsitch M, Cohen C, Mandelboim M, Gal Levin E, Rubin C, Indenbaum V, Tal I, Zavitan M, Zuckerman N, Bar-Chaim A, Kreiss Y, Regev-Yochay G. Covid-19 Breakthrough Infections in Vaccinated Health Care Workers. N Engl J Med. 2021 Jul 28:NEJM oa2109072. doi: 10.1056/NEJMoa2109072. Epub ahead of print. PMID: 34320281; PMCID: PMC8362591.

Lopez Bernal J, Andrews N, Gower C, Gallagher E, Simmons R, Thelwall S, Stowe J, Tessier E, Groves N, Dabrera G, Myers R, Campbell CNJ, Amirthalingam G, Edmunds M, Zambon M, Brown KE, Hopkins S, Chand M, Ramsay M. Effectiveness of Covid-19 Vaccines against the B.1.617.2 (Delta) Variant. N Engl J Med. 2021 Aug 12;385(7):585-594. doi: 10.1056/NEJMoa2108891. Epub 2021 Jul 21. PMID: 34289274; PMCID: PMC8314739.

Julie Grishaw, ACNP

Senior Editor, McGraw-Hill Education