COVID-19 Vaccine Breakthrough Cases: Data from the States

While COVID-19 vaccines are highly effective at preventing severe disease, hospitalization, and death from COVID-19 and also reduce the likelihood of mild or asymptomatic infection, a small share of fully vaccinated individuals do become infected, and some become hospitalized or have died. These rare occurrences are known as “breakthrough cases” which are to be expected, and historically known to occur with other vaccines as none is 100% effective.

The Centers for Disease Control and Prevention (CDC) currently monitors hospitalizations and deaths, from any cause, among fully vaccinated individuals with COVID-19, but not breakthrough infections, which it stopped monitoring as of May 1. CDC presents this data in aggregate at the national level but not by state, and there is no single, public repository for data by state or data on breakthrough infections, since the CDC stopped monitoring them.

We therefore reviewed the websites and other official state sources for all 50 states and D.C. to see which are providing data on COVID-19 breakthrough cases, hospitalizations and deaths, how regularly, and what those data may tell us. We only used data from official state sources (we did not include data available only in news media reports, for example). Where a state did not provide comparable data on overall COVID-19 cases, hospitalizations, or deaths reported over the period in which it captured breakthrough events, we obtained data on cases and deaths from the Johns Hopkins University COVID-19 Dashboard and on hospitalizations from the U.S. Department of Health & Human Services for the appropriate period (see methods for more detail).

Importantly, not all hospitalizations and deaths of those fully vaccinated and diagnosed with COVID-19 are due to COVID-19 or have a known cause at the time of reporting. The CDC reports that as of July 19, of 5,601 hospitalized breakthrough cases, 27% were asymptomatic or not related to COVID-19 and of 1,141 fatal cases, 26% were asymptomatic or not related to COVID-19. States differ in whether they provide this detail. DC, for example, reports that as of July 11, 50% of hospitalized breakthrough cases were due to COVID-19, 19% were not, and 31% were of unknown reason. However, few states made these distinctions. Where they did, we only included breakthrough hospitalizations and deaths due to COVID-19. In other cases, some of these breakthrough events may be due to causes other than COVID-19.

Overall, we find that:

• Half of states (25) report some data on COVID-19 breakthrough events (see Table 1). Twenty-four provide data on breakthrough cases, 19 on hospitalizations and on deaths.
• Fifteen of these states regularly update these data, often on a weekly basis. The rest use a different frequency, have one-time reports, have stopped updating, or have an unclear reporting frequency.
• The data reported from these states indicate that breakthrough cases, hospitalizations, and deaths are extremely rare events among those who are fully vaccinated against COVID-19 (see Figure 1). The rate of breakthrough cases reported among those fully vaccinated is below 1% in all reporting states, ranging from 0.01% in Connecticut to 0.54% in Arkansas.
  • The hospitalization rate among fully vaccinated people with COVID-19 ranged from effectively zero (0.00%) in California, Delaware, D.C., Indiana, New Jersey, New Mexico, Vermont, and Virginia to 0.06% in Arkansas. (Note: Hospitalization may or may not have been due to COVID-19.)
  • The rates of death among fully vaccinated people with COVID-19 were even lower, effectively zero (0.00%) in all but two reporting states, Arkansas and Michigan where they were 0.01%. (Note: Deaths may or may not have been due to COVID-19.)

• Almost all (more than 9 in 10) COVID-19 cases, hospitalizations, and deaths have occurred among people who are unvaccinated or not yet fully vaccinated, in those states reporting breakthrough data (see Figure 2).
  • The reported share of COVID-19 cases among those not fully vaccinated ranged from 94.1% in Arizona to 99.85% Connecticut.
  • The share of hospitalizations among those with COVID-19 who are not fully vaccinated ranged from in 95.02% in Alaska to 99.93% in New Jersey. (Note: Hospitalization may or may not have been due to COVID-19.)
  • The share of deaths among people with COVID-19 who are not fully vaccinated ranged from to 96.91% in Montana to 99.91% in New Jersey. (Note: Deaths may or may not have been due to COVID-19.)

Note that as more people get vaccinated, the share of cases, hospitalizations, and deaths accounted for by unvaccinated people will tend to fall, since there will be fewer unvaccinated people in the population. That will be true even if infection, hospitalization, and death from COVID-19 is still very rare among vaccinated people.

Implications

While information on breakthrough events is still limited and incomplete, this analysis of available state-level data indicates that COVID-19 breakthrough cases, and especially hospitalizations and deaths, among those who are fully vaccinated are rare occurrences in the United States. Moreover, this data indicate the vast majority of reported COVID-19 cases, hospitalizations, and deaths in U.S. are among those who are unvaccinated or not fully vaccinated. These findings echo the abundance of data demonstrating the effectiveness of currently authorized COVID-19 vaccines. Moving forward, particularly as the more transmissible Delta variant is now the dominant strain of COVID-19 circulating in the U.S., more robust state-level data will help to monitor ongoing vaccine effectiveness and inform discussions about booster vaccinations.