SARS-CoV-2 Vaccine Effectiveness against Omicron Variant in Infection-Naive Population, Australia, 2022

Area of Science:

• Immunology
• Epidemiology
• Public Health

Background:

• Western Australia experienced negligible SARS-CoV-2 transmission until the Omicron variant wave in February 2022.
• Over 90% of adults were vaccinated, offering a unique opportunity to assess vaccine effectiveness (VE) without prior infection immunity.
• This setting allowed for a clear evaluation of vaccine performance against the Omicron variant.

Purpose of the Study:

• To assess the real-world effectiveness of COVID-19 vaccines against the SARS-CoV-2 Omicron variant.
• To compare the duration of protection offered by different vaccine schedules.
• To determine the efficacy of vaccines in preventing infection, hospitalization, or death.

Main Methods:

• A case-control study matching 188,950 individuals with positive PCR tests to negative controls.
• Matching was based on age, week of test, and other potential confounding factors.
• Analysis focused on the period of February to May 2022 during the Omicron wave.

Main Results:

• Three-dose vaccine effectiveness was 42.0% against infection and 81.7% against hospitalization or death.
• A mixed vaccine schedule (viral-vectored primary series + mRNA booster) provided longer protection against infection than an all-mRNA schedule.
• Vaccines targeting the ancestral spike protein demonstrated approximately 80% effectiveness against severe outcomes from Omicron variant infections.

Conclusions:

• COVID-19 vaccines, particularly with a three-dose regimen, offer significant protection against severe outcomes from the Omicron variant.
• Mixed vaccine schedules may enhance the durability of protection against SARS-CoV-2 infection.
• These findings underscore the importance of vaccination in mitigating the impact of emerging variants.