Caveats on COVID-19 herd immunity threshold: the Spain case

Area of Science:

• Epidemiology
• Infectious Disease Modeling
• Public Health

Background:

• The COVID-19 pandemic highlights the critical need for herd immunity to prevent future outbreaks.
• Herd immunity thresholds are determined by the basic reproductive number (R0), influenced by virus characteristics, population, and environment.
• Accurate R0 estimation is complex, depending on methodology, data quality, and generation time.

Purpose of the Study:

• To analyze the challenges in estimating the basic reproductive number (R0) for SARS-CoV-2.
• To determine the herd immunity threshold (HIT) for the Spanish population.
• To assess the impact of different SARS-CoV-2 variants on R0 and HIT.

Main Methods:

• Review and reflection on R0 estimation methodologies.
• Calculation of R0 for the ancestral SARS-CoV-2 variant in Spain.
• Estimation of HIT based on a range of R0 values, considering variant evolution (e.g., Delta).

Main Results:

• R0 estimates for the ancestral SARS-CoV-2 variant in Spain ranged from 1.39 to 3.10.
• This resulted in a herd immunity threshold (HIT) between 28.1% and 67.7%.
• The Delta variant potentially increased R0 to 4.02-8.96, raising the HIT upper bound to 90%.

Conclusions:

• R0 estimation is sensitive to methodology, impacting herd immunity calculations.
• The emergence of variants like Delta significantly increases the required population immunity level.
• Achieving herd immunity in Spain necessitates adapting strategies to account for evolving viral transmission dynamics.