# Effectiveness of COVID-19 mRNA vaccine in preventing infection against Omicron strain: Findings from the Hiroshima Prefecture COVID-19 version J-SPEED for PCR center

## TL;DR

This study shows that mRNA vaccines offer some protection against Omicron infection, with higher effectiveness after completing the recommended vaccination schedule.

## Contribution

The study provides new evidence on the effectiveness of mRNA vaccines in preventing Omicron infections using a large dataset from Hiroshima Prefecture.

## Key findings

- Two-dose mRNA vaccines showed 34.7% effectiveness against Omicron infection after adjustment.
- Three-dose mRNA vaccines showed 26.4% effectiveness compared to two-dose recipients.
- Vaccination was associated with lower PCR test positivity rates compared to non-vaccinated individuals.

## Abstract

Despite the widespread adoption of various preventive measures since the spread of COVID-19, there remains a lack of consensus on universally acknowledged best practices. However, the significance of vaccination has risen to prominence as a paramount preventive strategy. Numerous investigations have demonstrated vaccine effectiveness against the omicron strain in severe disease and symptomatic disease, however, the scope of research pertaining to vaccine effectiveness in preventing infection is presently limited. Therefore, the current study aimed to evaluate COVID-19 mRNA (Pfizer-BioNTech BNT162b2 or Moderna mRNA-1273) vaccine effectiveness in preventing infection.

We conducted a test-negative case-control study using a dataset of 117,335 individuals. These data were collected through the COVID-19 J-SPEED form in the PCR center at Hiroshima Prefecture, Japan from 1 February to 17 March 2022. We estimated propensity score matching for vaccine status based on participants’ demographic characteristics. Subsequently, odds ratio was calculated from logistic regression to determine the association between vaccination status and test positivity rate adjusting for symptoms, exposure to close contact, and previous infection history. Vaccine effectiveness was defined as (1 –aORs) ×100%.

The PCR test positivity rates were 7.9%, 4.5%, and 2.8% for the non-vaccinated (non-vaccinated, vaccinated with a single dose, and vaccinated with two doses less than 14 days ago), vaccinated with two doses (vaccinated over 14 days ago), and three doses, respectively. In unadjusted and adjusted analyses, vaccine effectiveness of two doses against infection were 38.5% (95% confidence interval [CI]: 32.8%–43.8%) and 34.7% (95%CI: 28.4%–40.4%), respectively, compared to non-vaccinated group. Vaccine effectiveness of three doses were 33.8% (95%CI: 25.0%–41.5%) and 26.4% (95%CI: 16.4%–35.2%), respectively, compared to those vaccinated with two doses.

These results illustrate the protective effect of mRNA vaccines against Omicron strain infections and emphasize the significance of completing the suggested vaccination schedule.