Waning COVID-19 vaccine effectiveness in Japan

Area of Science:

• Epidemiology
• Immunology
• Public Health

Background:

• High COVID-19 vaccine uptake in Japan reached nearly 80% by late 2021.
• Concerns exist regarding the duration of vaccine-induced immunity against SARS-CoV-2.
• The emergence of mutated strains and public events like the Olympic Games complicate effectiveness assessments.

Purpose of the Study:

• To evaluate the waning effectiveness of COVID-19 vaccines in Japan.
• To determine the impact of time since vaccination on protection against COVID-19 transmission.
• To analyze factors influencing vaccine effectiveness, including viral variants and public health interventions.

Main Methods:

• Utilized time-series analysis to model the effective reproduction number (R(t)).
• Incorporated variables such as vaccine coverage (lagged), mutated strain prevalence, and an Olympic Games dummy variable.
• Controlled for environmental factors (temperature, humidity) and mobility data.

Main Results:

• Vaccine coverage more than 90 days prior significantly increased R(t).
• Effectiveness was completely diminished by 90 to 120 days post-vaccination.
• Significant waning of vaccine effectiveness was observed starting 90 days after the second dose.

Conclusions:

• COVID-19 vaccine effectiveness wanes substantially over time in the Japanese population.
• Immunity protection significantly decreases approximately 90 days after the second vaccine dose.
• Findings highlight the critical need for understanding and addressing vaccine durability, potentially through booster programs.