Important update: Healthcare facilities
CDC has updated select ways to operate healthcare systems effectively in response to COVID-19 vaccination. Learn more
To maximize protection from the Delta variant and prevent possibly spreading it to others, get vaccinated as soon as you can and wear a mask indoors in public if you are in an area of substantial or high transmission.
UPDATE
Given new evidence on the B.1.617.2 (Delta) variant, CDC has updated the guidance for fully vaccinated people. CDC recommends universal indoor masking for all teachers, staff, students, and visitors to K-12 schools, regardless of vaccination status. Children should return to full-time in-person learning in the fall with layered prevention strategies in place.
UPDATE
The White House announced that vaccines will be required for international travelers coming into the United States, with an effective date of November 8, 2021. For purposes of entry into the United States, vaccines accepted will include FDA approved or authorized and WHO Emergency Use Listing vaccines. More information is available here.
UPDATE
Travel requirements to enter the United States are changing, starting November 8, 2021. More information is available here.

The Possibility of COVID-19 after Vaccination: Breakthrough Infections

The Possibility of COVID-19 after Vaccination: Breakthrough Infections
The bottom line: COVID-19 vaccines protect people against severe illness, including disease caused by Delta and other variants circulating in the U.S.
  • COVID-19 vaccines protect people from getting infected and severely ill, and significantly reduce the likelihood of hospitalization and death.
  • Getting vaccinated is the best way to slow the spread of COVID-19 and to prevent infection by Delta or other variants.
  • A vaccine breakthrough infection happens when a fully vaccinated person gets infected with COVID-19. People with vaccine breakthrough infections may spread COVID-19 to others.
  • Even if you are fully vaccinated, if you live in an area with substantial or high transmission of COVID-19, you – as well as your family and community – will be better protected if you wear a mask when you are in indoor public places.
  • People who are immunocompromised may not always build adequate levels of protection after an initial 2-dose primary mRNA COVID-19 vaccine series. They should continue to take all precautions recommended for unvaccinated people, until advised otherwise by their healthcare professional. Further, CDC recommends that moderately to severely immunocompromised people receive an additional dose of vaccine.

COVID-19 vaccines are effective at preventing infection, serious illness, and death. Most people who get COVID-19 are unvaccinated. However, since vaccines are not 100% effective at preventing infection, some people who are fully vaccinated will still get COVID-19. An infection of a fully vaccinated person is referred to as a “vaccine breakthrough infection.”

What We Know about Vaccine Breakthrough Infections

  • Vaccine breakthrough infections are expected. COVID-19 vaccines are effective at preventing most infections. However, like other vaccines, they are not 100% effective.
  • Fully vaccinated people with a vaccine breakthrough infection are less likely to develop serious illness than those who are unvaccinated and get COVID-19.
  • Even when fully vaccinated people develop symptoms, they tend to be less severe symptoms than in unvaccinated people. This means they are much less likely to be hospitalized or die than people who are not vaccinated.
  • People who get vaccine breakthrough infections can be contagious.

CDC is collecting data on vaccine breakthrough infections and closely monitors the safety and effectiveness of all Food and Drug Administration (FDA)-authorized COVID-19 vaccines. Because vaccines are not 100% effective, as the number of people who are fully vaccinated goes up, the number of vaccine breakthrough infections will also increase. However, the risk of infection remains much higher for unvaccinated than vaccinated people. Studies so far show that vaccinated people are 8 times less likely to be infected and 25 times less likely to experience hospitalization or death. Vaccines remain effective in protecting most people from COVID-19 infection and its complications.

Vaccine Breakthroughs and Variants

CDC continues to actively monitor vaccine safety and effectiveness against new and emerging variants for all FDA-authorized COVID-19 vaccines. Research shows that the FDA-authorized vaccines offer protection against severe disease, hospitalization, and death against currently circulating variants in the United States. However, some people who are fully vaccinated will get COVID-19.

The Delta variant is more contagious than previous variants of the virus that causes COVID-19. However, studies so far indicate that the vaccines used in the United States work well against the Delta variant, particularly in preventing severe disease and hospitalization. Overall, if there are more infections with SARS-CoV-2 (the virus that causes COVID-19) there will be more vaccine breakthrough infections. However, the risk of infection, hospitalization, and death are all much lower in vaccinated compared to unvaccinated people. Therefore, everyone aged 12 years and older should get vaccinated to protect themselves and those around them, including family members who are not able to be vaccinated from severe disease and death.

How CDC Monitors Breakthrough Infections

CDC has multiple surveillance systems and ongoing research studies to monitor the performance of vaccines in preventing infection, disease, hospitalization, and death.  CDC also collects data on vaccine breakthrough infections through outbreak investigations. One important system that CDC uses to track vaccine breakthrough infections is COVID-NET (the Coronavirus Disease 2019 [COVID-19]-Associated Hospitalization Surveillance Network). This system provides the most complete data on vaccine breakthrough in the general population. COVID-NET is a population-based surveillance system that collects reports of lab-confirmed COVID-19-related hospitalizations in 99 counties in 14 states. COVID-NET covers approximately 10% of the U.S. population. One recent COVID-NET publication assessed the effectiveness of COVID-19 vaccines in preventing hospitalization among adults ≥ 65 years. This system provides complete data on vaccine breakthrough hospitalizations in the general population.

Examples of CDC’s systems for monitoring include:
Outcome monitored Population monitored Monitoring system
Infection Long-term care facility residents NHSN
Infection and symptomatic illness Healthcare providers and frontline workers HEROES/RECOVER
Hospitalization and deaths Hospitalized adults IVY
Hospitalization and deaths Hospitalized people (all ages) COVID-NET
Urgent care, emergency care, hospitalization and deaths Urgent care, emergency departments, and hospitalized people (all ages) VISIONpdf icon

Voluntary Reporting by State Health Departments

When the United States began widespread COVID-19 vaccination, CDC put in place a system where state health departments could report COVID-19 vaccine breakthrough infections to CDC.  On May 1, 2021, after collecting data on thousands of vaccine breakthrough infections, CDC changed the focus of how it uses data from this reporting system. One of the strengths of this system is collecting data on severe cases of vaccine breakthrough COVID-19 since it is likely that most of these types of vaccine breakthrough cases seek medical care and are diagnosed and reported as a COVID-19 case. Persons with asymptomatic or mild cases of vaccine breakthrough infections may not seek testing or medical care and thus these types of vaccine breakthrough cases may be underrepresented in this system. For this reason, CDC relies on a variety of additional surveillance approaches to ensure that it is collecting information on all types of vaccine breakthrough cases. CDC continues to monitor data on all cases reported by state health departments as vaccine breakthrough cases. Currently, 49 states have reported at least one vaccine breakthrough infection to this system.

CDC monitors reported data on hospitalized and fatal vaccine breakthrough cases to understand:

  • Patterns by age and sex.
  • The specific types or brands of vaccine involved.
  • Underlying health conditions in these persons.
  • Which SARS-CoV-2 variants are observed in persons who are hospitalized or who die.