The risk of COVID-19 hospitalization among previously infected adults is 89% to 90% lower for at least 10 months than for their never-infected peers, according to a meta-analysis published late last week in The Lancet.
In the largest meta-analysis on the extent of infection-conferred protection against four SARS-CoV-2 strains, the COVID-19 Forecasting Team analyzed 65 studies from 19 countries published up to Sep 30, 2022, that estimated the risk of infection-related hospitalization among participants who had or hadn't had a previous case.
Protection against Omicron wanes faster
"Our 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," the authors wrote.
Specifically, pooled effectiveness against Omicron reinfection was 45.3%, while it was 44.0% effective against symptomatic illness caused by that variant. Average pooled effectiveness was higher than 78% against hospitalization and death for all variants, including Omicron.
By 10 months, protection against reinfection had waned against the wild-type, Alpha, and Delta variants but was still 78.6%. In contrast, protection against Omicron reinfection fell to 36.1% over the same period. But protection against hospitalization or death remained high at 10 months for all variants, at 90.2% for the wild-type, Alpha, and Delta variants and 88.9% for Omicron.
The authors said the results suggest that the level and length of infection-conferred protection against reinfection, symptomatic illness, and severe disease is at least as good as that provided by two doses of the Pfizer/BioNTech and Moderna mRNA vaccines for the wild-type, Alpha, Delta, and Omicron BA.1 variants (the study took place before the emergence of the more transmissible Omicron XBB strain and its subvariants).
Vaccination still recommended
But they added that vaccination is still recommended, even among previously infected adults.
"Vaccination is the safest way to acquire immunity, whereas acquiring natural immunity must be weighed against the risks of severe illness and death associated with the initial infection," senior author Stephen Lim, PhD, of the Institute for Health Metrics and Evaluation (IHME) at the University of Washington, said in a Lancet news release.
First author Caroline Stein, DDS, PhD, of IMHE, said that vaccines remain important for everyone to protect high-risk groups such as older than 60 years and those with underlying illnesses.
"This also includes populations that have not previously been infected and unvaccinated groups, as well as those who were infected or received their last vaccine dose more than six months ago," she said in the release. "Decision makers should take both natural immunity and vaccination status into consideration to obtain a full picture of an individual's immunity profile."
Role of continued variant assessment
Coauthor Hasan Nassereldine, PhD, of IMHE, said in the release that the weaker cross-variant immunity with Omicron and its substrains reflects the variants' mutations, which help them escape immunity more easily than other lineages.
Vaccines remain important for everyone to protect high-risk groups such as older than 60 years and those with underlying illnesses.
"The limited data we have on natural immunity protection from the Omicron variant and its sub-lineages underscores the importance of continued assessment, particularly since they are estimated to have infected 46% of the global population between November 2021 and June 2022," he said. "Further research is also needed to assess the natural immunity of emerging variants and to examine the protection provided by combinations of vaccination and natural infection."
In a related commentary, Cheryl Cohen, MBBS, DPhil, of the National Institute for Communicable Diseases, and Juliet Pulliam, PhD, of Stellenbosch University, both in South Africa, noted that the global percentage of people who had SARS-CoV-2 antibodies by September 2021was estimated at 59%, with substantial variation in the proportion of those with immunity from infection or vaccination in different settings.
"High levels of immunity are an important contributor to the lower levels of severity observed with infection caused by emerging omicron subvariants," they wrote.
"As SARS-CoV-2 epidemiology shifts to more stable circulation patterns in the context of high levels of immunity, studies of the burden and cost of SARS-CoV-2 infection and risk groups for severe disease are needed to guide rational vaccination policy and decisions around prioritisation in relation to other vaccine-preventable diseases."