Investigating Breakthrough Infections

COVID-19 vaccines are effective and are a critical tool to help bring the pandemic under control. However, no vaccines are 100% effective at preventing illness in vaccinated people. There will be a small percentage of fully vaccinated people who still get sick, be hospitalized, or die from COVID-19. As with other vaccines, this is expected. As the number of people who are vaccinated goes up, the number of breakthrough cases is also expected to increase.

Vaccine breakthrough cases may occur for many reasons:

• Variants of the virus that causes COVID-19 could emerge that are less vulnerable to available vaccines.

• Protection (immunity) in vaccinated people could lessen over time.

• Specific people may develop less immunity after vaccination.

CDC will work with state, tribal, local and territorial authorities to identify and investigate specific institutional and facility outbreaks, for example specific outbreaks in shared or congregate housing. They will assess whether any of the reasons listed above play a role in why vaccinated people might still get COVID-19.

CDC is also working with state and territorial health departments to investigate vaccine breakthrough cases. This includes monitoring trends in case character­istics, such as age or vaccination status, and virus variants identified from people with these infections. Investigations of breakthrough cases help support the vaccine effectiveness studies under way. Cases of vaccine breakthrough that occur within vaccine effectiveness studies are also examined.

National Surveillance of Vaccine Breakthrough Cases

When COVID-19 vaccines were being introduced, CDC created a vaccine breakthrough surveillance system to allow state and local health departments to voluntarily report vaccine breakthrough cases to CDC. State and local health departments continue to report breakthrough cases to CDC to identify and investigate patterns or trends among hospitalized or fatal vaccine breakthrough cases. CDC will continue this activity until more sustainable systems for monitoring breakthrough cases are fully established.

CDC is also working with the Emerging Infections Program (EIP) sites in nine states to obtain genetic sequence data (like a DNA fingerprint) for the virus that causes COVID-19 from vaccine breakthrough cases. Those data will include asymptomatic and mild vaccine breakthrough cases and compare those to COVID-19 cases among unvaccinated people.

See the latest data on hospitalized or fatal vaccine breakthrough cases.

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