FRIDAY, Feb. 17, 2023 (HealthDay News) — Natural immunity is substantially lower for the omicron BA.1 COVID-19 variant compared with previous variants and declines more rapidly over time, according to a systematic review and meta-analysis published online Feb. 16 in The Lancet.

Caroline Stein, Ph.D., from the University of Washington in Seattle, and colleagues from the COVID-19 Forecasting Team conducted a systematic literature review to estimate protection from past infection by variant and by time since infection.

Based on data from 65 studies (up to the end of September 2022; 19 countries), the researchers found that protection from past infection and any symptomatic disease was high for ancestral, alpha, beta, and delta variants but was substantially lower for the omicron BA.1 variant. For symptomatic disease, pooled effectiveness against reinfection by the omicron BA.1 variant was 45.3 percent, while the pooled effectiveness against omicron BA.1 symptomatic disease was 44.0 percent . For severe disease, mean pooled effectiveness was >78 percent for all variants, including omicron BA.1. Over time, protection from reinfection from ancestral, alpha, and delta variants declined but remained at 78.6 percent at 40 weeks. For the omicron BA.1 variant, protection against reinfection declined more rapidly and was estimated at 36.1 percent at 40 weeks. However, protection against severe disease remained high for all variants (90.2 percent for ancestral, alpha, and delta variants and 88.9 percent for omicron BA.1 at 40 weeks).

“The immunity conferred by past infection should be weighed alongside protection from vaccination when assessing future disease burden from COVID-19, providing guidance on when individuals should be vaccinated, and designing policies that mandate vaccination for workers or restrict access, on the basis of immune status, to settings where the risk of transmission is high, such as travel and high-occupancy indoor settings,” the authors write.

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