Prior Infection Provides Protection Against Severe COVID for Up to 10 Months, Regardless of the Variant

Natural immunity acquired from a COVID infection provides strong and lasting protection against severe illness upon reinfection for up to 10 months, according to a new study published in The Lancet. This protection held regardless of the variant, although it did wane more quickly with Omicron, which was the dominant COVID strain at the time of the study, researchers found.

Looking at pooled data from 65 studies conducted in 19 countries, researchers found that protection against hospitalization and death remains at 89% for Omicron and 90% for earlier variants for up to 10 months post-infection.

Protection against reinfection waned with the Omicron variant; after 10 months, a previous Omicron infection provides only 36% protection against a follow-up COVID infection. 

Importantly, the level of protection provided by natural immunity is as durable and similar in magnitude to the protection provided by two doses of the mRNA vaccines developed by Moderna and Pfizer-BioNTech, the authors wrote.

For the study, researchers searched peer-reviewed publications, reports, preprints, medRxiv, and news articles in databases, such as PubMed, medRxiv, and Web of Science using keywords, such as SARS-CoV-2, a previous or past infection, natural immunity, or re-infection. 

They accrued data from retrospective and prospective cohort studies and case-control studies with test-negative designs. The extracted data included primary and re-infection SARS-CoV-2 variant (ancestral, pre-Delta, Delta, or Omicron), infection outcomes (symptomatic/severe), age, the magnitude of protection (high/low), the median time since infection, time since baseline in weeks, and the test/assay used to determine past infection.

The studies that met the inclusion criteria used antibody testing, reverse transcription-polymerase chain reaction (RT-PCR), rapid antigen test (RAT), or a combination of these to determine past infection status. 

The researchers defined study outcome(s), re-infection as a positive test more than 90 days after a past positive test, two positive tests after four negative tests, or a positive with a positive antibody test. Likewise, they defined symptomatic re-infection as re-infection with SARS-CoV-2 that leads to new onset of symptoms and severe illness as re-infection that leads to hospitalization or death. 

Using the criteria, 65 studies were identified from 19 different countries. The meta-analyses showed 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.

Mean pooled effectiveness was greater than 78% against severe disease (hospitalization and death) for all variants, including omicron BA.1. Protection from re-infection from ancestral, alpha, and delta variants declined over time but remained at 79% at 40 weeks. Protection against severe disease remained high for all variants, with 90% for ancestral, alpha, and delta variants, and 89% for omicron BA.1 at 40 weeks.

Pooled effectiveness against re-infection by the omicron BA.1 variant was 45% and 44% against omicron BA.1 symptomatic disease. Protection against re-infection by the omicron BA.1 variant declined rapidly and was estimated at 36% at 40 weeks. 

The authors note this study carries significant policy implications. First, the findings suggest that surveillance systems monitoring SARS-CoV-2 re-infections and variant emergence remain relevant as they could help manage current and future transmission. Second, nations should consider restricting travel or access to specific venues based on immune status conferred by both vaccination and natural infection. Third, past infection-acquired immunity should be strongly considered when prioritizing people for COVID-19 vaccination, including boosters. Finally, when new variants emerge, epidemiological studies should monitor the protection afforded by past infections and not just the COVID-19 vaccination.