Effectiveness of mRNA COVID-19 Booster Vaccines against Omicron and Delta Variants among US Veterans

Abstract

Background: Effectiveness of mRNA vaccinations against Omicron among US veterans is unknown. Objectives: To estimate mRNA vaccine effectiveness (VE) as measured by risk of COVID infection. Design, Setting, and Participants: This matched test negative case control study was conducted using COVID test results at Veterans Health Administration sites from December 1, 2021, to December 31, 2021. VE against COVID infection and COVID related hospitalization and death were estimated using electronic health records from veterans who routinely sought care at a VHA facility and had a test result positive for COVID (cases) or negative for COVID (controls). Data were analyzed from December 1, 2021 to January 14, 2022. Exposures: Vaccination status, defined as number of mRNA vaccine doses (1, 2, 3 or more) as the primary explanatory variable, at time of test. Main Outcomes and Measures: The main outcome of interest was a positive result for COVID on a polymerase chain reaction or antigen test. Secondary outcomes included COVID related hospitalization and death, defined by discharge data and proximity of event to positive test result. VE was estimated from odds ratios for COVID infection with 95% CIs. Results: VE for those who have received a mRNA vaccine booster is 77% (95%, 75 to 79) against Delta while 62% (95%, 59 to 65) against Omicron infection, 91% (95%, 85 to 94) against COVID related hospitalization with better VE against Omicron, and 96% (95%, 91 to 98) against COVID related death with no discernable difference between Omicron and Delta. However, without a booster, VE against COVID related death decreases to 76% (95%, 62 to 85). Conclusions: Although Omicron is highly contagious and more likely to result in breakthrough infections against the current mRNA vaccines, a mRNA vaccine booster is still effective against severe COVID related outcomes.

Clinical severity of, and effectiveness of mRNA vaccines against, covid-19 from omicron, delta, and alpha SARS-CoV-2 variants in the United States: prospective observational study

“…Emerging vaccine effectiveness estimates globally suggest reduced effectiveness against the omicron variant compared with previous variants,282930 including an estimate of 70% vaccine effectiveness for two doses of the BNT162b2 vaccine to prevent hospital admissions due to the omicron variant in South Africa in November-December 2021 9. Using electronic health record data from sites across the US, the VISION Network recently estimated mRNA vaccine effectiveness against hospital admissions due to the omicron variant to be to 52% for two vaccine doses, with the second dose received within 180 days before illness onset, 38% for two doses received more than 180 days before illness onset, and 82% for three vaccine doses 11.…”

Clinical Severity and mRNA Vaccine Effectiveness for Omicron, Delta, and Alpha SARS-CoV-2 Variants in the United States: A Prospective Observational Study

“…Emerging vaccine effectiveness estimates globally suggest reduced effectiveness against Omicron compared with prior variants [28–30], including an estimate of 70% vaccine effectiveness for two doses of the BNT162b2 vaccine to prevent Omicron hospitalizations in South Africa in November-December 2021 [9]. Using electronic health record data from sites across the United States, the VISION Network recently estimated mRNA vaccine effectiveness against Omicron hospitalizations to be to 52% for two vaccine doses with the second dose received within 180 days before illness onset, 38% for 2 doses received >180 days before illness onset, and 82% for 3 vaccine doses [11].…”

Effectiveness of Covid-19 vaccines against SARS-CoV-2 Omicron variant (B.1.1.529): A systematic review with meta-analysis and meta-regression

“…Full dose vaccination represents 2 doses of BNT162b2, ChAdOx1 nCov-19 and mRNA-1273 vaccines, or 1 dose of Ad26.COV2.S vaccines, whereas booster dose vaccination was defined as the administration of an extra dose of Covid-19 vaccine on top of the full dose vaccination. Two studies 31,37 did not specify the type of vaccines being used; therefore, in this study, we defined it as a mixture. However, it does not necessarily mean that the vaccines are heterologous or mix-and-matched.…”

“…If the study did not report the limit of follow up interval, then it would be written as not reported. A detailed outcome summary with 95%CI is compiled in Table S4 and 31,37 did not specify the type of vaccines being used; therefore, in this study, we defined it as a mixture. However, it does not necessarily mean that the vaccines are heterologous or mix-and-matched.…”