Covid-19 Rates by Time since Vaccination during Delta Variant Predominance

Abstract

Background: With the emergence of the delta variant, the United States experienced a rapid increase in Covid-19 cases in 2021. We estimated the risk of breakthrough infection and death by month of vaccination as a proxy for waning immunity during a period of delta variant predominance.

Methods: Covid-19 case and death data from 15 U.S. jurisdictions during January 3 to September 4, 2021 were used to estimate weekly hazard rates among fully vaccinated persons, stratified by age group and vaccine product. Case and death rates during August 1 to September 4, 2021 were presented across four cohorts defined by month of vaccination. Poisson models were used to estimate adjusted rate ratios comparing the earlier cohorts to July rates.

Results: During August 1 to September 4, 2021, case rates per 100,000 person-weeks among all vaccine recipients for the January to February, March to April, May to June, and July cohorts were 168.8 (95% confidence interval [CI], 167.5 to 170.1), 123.5 (95% CI, 122.8 to 124.1), 83.6 (95% CI, 82.9 to 84.3), and 63.1 (95% CI, 61.6 to 64.6), respectively. Similar trends were observed by age group for BNT162b2 (Pfizer-BioNTech) and mRNA-1273 (Moderna) vaccine recipients. Rates for the Ad26.COV2.S (Janssen-Johnson & Johnson) vaccine were higher; however, trends were inconsistent. BNT162b2 vaccine recipients 65 years of age or older had higher death rates among those vaccinated earlier in the year. Protection against death was sustained for the mRNA-1273 vaccine recipients. Across age groups and vaccine types, people who were vaccinated 6 months ago or longer (January-February) were 3.44 (3.36 to 3.53) times more likely to be infected and 1.70 (1.29 to 2.23) times more likely to die from COVID-19 than people vaccinated recently in July 2021.

Conclusions: Our study suggests that protection from SARS-CoV-2 infection among all ages or death among older adults waned with increasing time since vaccination during a period of delta predominance. These results add to the evidence base that supports U.S. booster recommendations, especially for older adults vaccinated with BNT162b2 and recipients of the Ad26.COV2.S vaccine. (Funded by the Centers for Disease Control and Prevention.).

Figure 1.

Weekly Covid-19 Hazard Rates for Cases and Deaths by Vaccination Cohort for 14 to 15 U.S. Jurisdictions, from January 3 to September 4, 2021. Weekly Covid-19 hazard rates are presented for A) cases and B) deaths by vaccination cohort. The 95% confidence intervals are presented around hazard rates. The widths of the intervals have not been adjusted for multiplicity and any of the inferences drawn may not be reproducible. Fifteen jurisdictions were included for cases and 14 jurisdictions were included for deaths (Wisconsin was not included in death estimates). Persons vaccinated with the Ad26.COV2.S vaccine (Janssen–Johnson & Johnson) in the January to February cohort and all persons 12 to 17 years of age vaccinated either with the mRNA-1273 (Moderna) or Ad26.COV2.S vaccine were excluded.

Figure 2.

Covid-19 Case Rates by Vaccination Cohort and Age for the BNT162b2, mRNA-1273, and Ad26.COV2.S Vaccines for 15 U.S. Jurisdictions, from August 1 to September 4, 2021. Covid-19 case rates are presented by vaccination cohort and age for the A) BNT162b2 (Pfizer–BioNTech), B) mRNA-1273 (Moderna), and C) Ad26.COV2.S (Janssen–Johnson & Johnson) vaccines. Error bars represent 95% confidence intervals. The widths of the intervals have not been adjusted for multiplicity and any of the inferences drawn may not be reproducible. Persons vaccinated with the Ad26.COV2.S vaccine in the January to February cohort and all persons 12 to 17 years of age vaccinated with either the mRNA-1273 or Ad26.COV2.S vaccine were excluded.

Figure 3.

Covid-19 Death Rates by Vaccination Cohort and Age for the BNT162b2, mRNA-1273, and Ad26.COV2.S Vaccines for 14 U.S. Jurisdictions, from August 1 to September 4, 2021. Covid-19 death rates are presented by vaccination cohort and age for the A) BNT162b2 (Pfizer–BioNTech), B) mRNA-1273 (Moderna), and C) Ad26.COV2.S (Janssen–Johnson & Johnson) vaccines. Only 14 jurisdictions contributed deaths. Wisconsin did not contribute death data due to lack of linkage between vital records and COVID-19 case surveillance. Error bars represent 95% confidence intervals. The widths of the intervals have not been adjusted for multiplicity and any of the inferences drawn may not be reproducible. Persons vaccinated with the Ad26.COV2.S vaccine in the January to February cohort and all persons 12 to 17 years of age vaccinated with either the mRNA-1273 or Ad26.COV2.S vaccine were excluded. *For a death count that is 15 or fewer, the rate may be unstable and should be interpreted with caution.