On June 29, 2021, we reported that 0.049% of patients fully vaccinated for COVID-19 developed a breakthrough COVID infection.1 This data included 8,662,367 fully vaccinated patients, of which 4,260 had a breakthrough COVID case. Patients were considered “fully vaccinated” 14 days after completing a manufacturer-recommend COVID-19 vaccination series (i.e., 14 days after the second dose of Pfizer-BioNTech, the second dose of Moderna, or the first dose of J&J/Janssen). Breakthrough cases were defined as those with a positive PCR-based SARS-CoV-2 test result and/or a COVID-19 diagnosis at least 14 days following the final dose in the series. The initial data included patients who were fully vaccinated on or before May 9, 2021.
As of October 13, 2021, the rate of breakthrough cases has risen to 1.15%. Out of the 19,514,682 patients who are fully vaccinated, 223,965 had a breakthrough case (1.15%), 22,822 were hospitalized (0.12%), and 2,752 died (0.014%). This is a similar rate of breakthrough hospitalizations and deaths as we reported in June—about 10% of breakthrough cases are hospitalized, and about 10% of those hospitalized die in the hospital.
Patients over age 65 were more likely to have breakthrough cases and more likely to require hospitalization, as shown in Figure 2. The median age of patients with breakthrough cases was 57 years, whereas the median age for patients with breakthrough hospitalizations or in-hospital death was 72 years and 76 years, respectively.
While the proportion of breakthrough cases is still very low, we are seeing an increasing rate of breakthrough cases since July 2021, as shown in Figure 3. One reason for the increased breakthrough rate is the current predominance of the delta variant, which is more contagious.2 Another likely reason for the increase is waning vaccination immunity in our patient population.3,4 This study excludes patients who had a booster vaccine. We expect that as boosters become more prevalent, we will see fewer breakthrough cases.
We wondered whether the recent increase in breakthrough cases was primarily due to waning immunity or the changing patterns in viral spread starting around July 2021. Figure 4 shows the number of weeks after the last dose of vaccine that a breakthrough case, hospitalization, or death occurred. We see the greatest number of breakthroughs between 22 and 27 weeks (or about 5-6 months) after completing a COVID-19 vaccine series. The decline in number of breakthroughs greater than 30 weeks after vaccination can be attributed to the smaller number of vaccinated individuals that had their last vaccine dose prior to April 2021, as the vaccine was not yet widely available at that time.
Taking figures 3 and 4 together, it appears that the rate of increase in breakthroughs is correlated both with the emergence of delta as the predominant variant and with waning vaccine immunity. Local rates of social distancing and masking also affect these numbers, as do variations in local vaccination rates and the number of COVID cases overall.
The Wall Street Journal also published on these findings; read their article here.
