Assessing Breakthrough Infections Associated with the AZD1222 Vaccine


Investigators examined asymptomatic infection and the duration of viral shedding in symptomatic breakthrough infections in a phase 3 study with the AZD1222 COVID-19 vaccine.

astrazeneca vaccine

AstraZeneca’s AZD1222 COVID-19 vaccine has been undergoing various studies for efficaciousness and safety. Thus far, the vaccine has been approved for use internationally in various countries. The company, however, has not submitted its clinical data to the Food and Drug Administration for potential approval here in the United States.

In a study presented virtually at IDWeek, investigators examined asymptomatic infection and length of viral shedding in symptomatic breakthrough infections in a phase 3 study with the AZD1222 COVID-19 vaccine. 

In the study, the AZD1222 vaccine was delivered in 2 doses at 4 weeks apart. It was shown to have a vaccine efficacy (VE) of 74% against symptomatic COVID-19, which was RT-PCR-confirmed. The study was 2:1 randomized, placebo-controlled, and was performed in the US, Chile, and Peru with a total of 32,451 participants.

Overall, 358 participants had breakthrough infections. “Incidences per 1000 person-years of symptomatic infections were 25.62 for AZD1222 vs 103.42 for placebo (VE 75.23%; 95% CI 65.33, 82.31) and of asymptomatic infections were 51.24 vs 111.95 (VE 54.24%; 95% CI 39.99, 65.10).”

The median time to viral clearance for symptomatic infections was 11 days in those vaccinated vs 16 days in the placebo cohort.

Investigators found utility in the vaccine in terms of reducing transmission. “AZD1222 resulted in lower yet meaningful VE against asymptomatic compared to symptomatic infections, as determined by nucleocapsid (N) seroconversion, and shortened viral shedding in symptomatic SARS-CoV-2 breakthrough infections vs placebo, highlighting its potential contribution to reducing viral transmission,” the investigators reported in terms of their findings.

The study, “Asymptomatic Infection and Duration of Viral Shedding in Symptomatic Breakthrough Infections in a Phase 3 Study of AZD1222 (ChAdOx1 nCoV-19),” was presented virtually at IDWeek 2021, held September 29-October 3, 2021

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