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Monday, January 2, 2023

Effectiveness of the Coronavirus Disease 2019 (COVID-19) Bivalent Vaccine

Key Findings

Among 51011 working-aged Cleveland Clinic employees, the bivalent COVID-19 vaccine booster was 30% effective in preventing infection, during the time when the virus strains dominant in the community were represented in the vaccine.
Compared to last exposure to SARS-CoV-2 within 90 days, last exposure 6-9 months previously was associated with twice the risk of COVID, and last exposure 9-12 months previously with 3.5 times the risk.

The Omicron variant in December 2021 brought a significant change to the immune protection landscape.

Previously infected or vaccinated individuals were no longer protected from COVID-19
Vaccine boosting provided some protection against the OmicRON variant
The degree of protection was not near that of the original vaccine against the pre-Omicron variants of SARS-CoV-2
Prior infection with an earlier lineage protected against subsequent infection with a subsequent lineage, but this protection appeared to wear off within a few months

Study design

A retrospective cohort study conducted at the Cleveland Clinic Health System (CCHS) in the United States.

Setting

Systems were designed to enable Occupational Health to interview and remotely monitor symptoms for all employees while the latter were isolated at home
Voluntary vaccination for COVID-19 began on 16 December 2020, and the monovalent vaccine as a booster became available to employees on 5 October 2021
The bivalent version of the vaccine began to be offered to employees in September 2022

Variables

Age, gender, job location, and job type categorization into clinical or non-clinical
Prior COVID-19 was defined as a positive NAAT for SARS-CoV-2 any time before the study start date
Date of infection for a prior episode was the date of the first positive test for that episode
Subsequent positive tests within 90 days were considered part of the same episode of illness
Most of the positive tests during the study period would have been tests done to evaluate suspicious symptoms, some to evaluate known exposures, and some as part of pre-operative or pre-procedural screening

Statistical Analysis

A Simon-Makuch hazard plot was created to compare the cumulative incidence of COVID-19 in the bivalent vaccinated and non-vaccinated states
Bivalent vaccination was included as a time-dependent covariate as the primary model included all study subjects and a secondary model included only those with prior exposure to SARS-CoV-2 by infection or vaccination
Vaccine effectiveness was calculated from the hazard ratios for bivalent vaccination in the models

Results

Out of 51977 eligible subjects, 966 (1.9%) were excluded because of missing age or gender.
Of the remaining 51011 employees included in the study, 34507 (68%) had been in employment since before the onset of the COVID-19 pandemic (pre-pandemic hires).
1794 subjects (3.5%) were censored during the study period because of the study.

Baseline characteristics

This was a relatively young population, with a mean age of 42 years.

Risk of COVID-19 based on prior infection and vaccination history

Pre-Delta: those last infected during the pre-Delta or Delta phase
Omicron BA.1/BA.2 phase
Last Infected During the Omnicon BA.4/BA5 phase- decreasing order of risk
Number of vaccine doses: higher the number of vaccines previously received, the higher the risk of contracting COVID

Bivalent vaccine effectiveness

In a multivariable *** proportional hazards regression model adjusted for age, gender, hire cohort, job category, number of COVID-19 vaccine doses prior to study start, and epidemic phase, the vaccine effectiveness was 30% (95% CI, 30% - 39% CI).
The more recent the last prior COVID episode was, the lower the risk of infection, and the greater the number of vaccine doses previously received, the higher the risk.

Bivalent vaccine effectiveness among those with prior SARS-CoV-2 infection or vaccination

One way to assess the effectiveness of a vaccine is to adjust for time since the proximate source of the infection/vaccination.
Among persons with prior exposure to SARS by infection/ vaccination, the hazard ratios for bivalent vaccination after adjusting for time after exposure are shown in table 3
In addition to the 21% protective effect of bivalent, those with last exposure 6-9 months previously have twice the risk of COVID-19 compared to those with no previous exposure.

The study found that the current bivalent vaccines were about 30% effective overall in protecting against infection with SARS-CoV-2, when the Omicron BA.4/BA.5 lineages were the predominant circulating strains.

The magnitude of protection afforded by bivalent vaccination was similar to that estimated in a recent study using data from the Increasing Community Access to Testing (ICATT).
There are several limitations to this study
Large sample size
Some individuals with unrecognized prior infection would have been misclassified as previously uninfected, which could have resulted in underestimating the protective effect of the vaccine
Those who chose to receive the bivalent vaccine might have been more likely to get tested for the same symptoms after getting the vaccine (bivalent vaccinated state), thereby disproportionately detecting more incident infections among those who received the vaccine

Author contributions

N. S. S.: Conceptualization, methodology, validation, investigation, data curation, software, formal analysis, visualization, writing-reviewing and editing

REFERENCES

****** FP, Thomas SJ, Kitchin N, et al. Safety and effectiveness of the BNT162b2 mRNA Covid-19 vaccine against SARS-CoV-2 infections and COVID-19 cases, hospitalisations, and deaths following a nationwide vaccination campaign in Israel: an observational study using national surveillance data. N Engl J Med 2020; 383:2603-2615.CrossRefPubMed
Baden LR, El Sahly HM, Essink B, and Al-Haas EJ. Efficacy and safety of the mRNA-1273, or “MNC-19,” vaccine
Shrestha NK, Burke PC, Nowacki AS, Terpeluk P, Gordon SM
Necessity of the vaccine in persons who have already been vaccinated
Protection against the Omicron variant from previous SARS infection
Rapid waning of protection induced by prior BA.1/BA.2 infection against BA.617
Genomic surveillance of SARS variants
Predominance of the Delta (B.1.1, BA.529) Variants
United States, June 2021-January 2022. MMWR Morb Mortal Wkly Rep 2022; 71.1101/2021.
Available at: https://www.cdc.gov/mmwr/volumes/71/wr/412-

https://www.medrxiv.org/content/10.1101/2022.12.17.22283625v1.full