Measuring vaccine protection when the population is mostly vaccinated

Area of Science:

• Epidemiology
• Immunology
• Public Health

Background:

• Classical vaccine effectiveness (VE) measures face limitations in assessing vaccine protection.
• These limitations are particularly evident in populations with high vaccination rates or prior infections.

Purpose of the Study:

• To introduce and demonstrate the utility of the adjusted number of cases (ANC) as a foundational metric for vaccine effectiveness.
• To address biases associated with small or unrepresentative unvaccinated reference groups and incomplete data.
• To evaluate the effectiveness of COVID-19 booster doses against severe disease using real-world data.

Main Methods:

• Proposed the adjusted number of cases (ANC) as a novel building block for vaccine effectiveness measures.
• Utilized ANC to account for biases in traditional VE calculations.
• Applied these derived measures to assess booster dose protection against severe COVID-19 in Israel.

Main Results:

• The ANC-derived measures offer a more comprehensive understanding of population immunity compared to traditional VE.
• This approach facilitates meaningful comparisons across different vaccination statuses.
• Insights gained inform public health policy decisions regarding vaccination strategies.

Conclusions:

• Traditional VE measures have reduced informative value in contexts of high vaccination and infection prevalence.
• The adjusted number of cases (ANC) provides a more robust and insightful assessment of vaccine effectiveness.
• Complementary measures like ANC are crucial for guiding public health strategies, as shown by the evaluation of COVID-19 booster efficacy.