The largest meta-analysis ever done of its kind shows that the natural immunity provided by Covid-19 infection protects a person against severe illness just as well as two doses of the mRNA vaccine.
The scientists involved in the study, published in The Lancet, said that people who had been infected with Covid reduced their chances of hospitalisation and death by 88% over 10 months compared with somebody who had not been infected.
Natural immunity provided by infection was shown to be “at least as high, if not higher” than the immunity provided by two doses of Moderna or Pfizer mRNA vaccines against the ancestral, Alpha, Delta and Omicron BA.1 variants, reports Medscape.
But protection against the BA.1 subvariant of Omicron was not as high – 36% at 10 months after infection, said the research team from the Institute for Health Metrics and Evaluation at the University of Washington.
They examined 65 studies from 19 countries up until 31 September 2022, but did not study data about infection from Omicron XBB and its sub-lineages. People who had immunity from both infection and vaccination, known as hybrid immunity, were not studied.
However, they said, the findings don’t mean people should skip the vaccines and rather get Covid deliberately.
“The problem of thinking you will get immunity that way is you might just be one of those people who end up in the hospital or die,” said Dr Christopher Murray, DPhil, director of the IHME. “Why would you take the risk when you can get immunity through vaccination quite safely?”
“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,” said lead author Dr Stephen Lim from the IHME.
And IHME co-author Dr Caroline Stein said: “Vaccines continue to be important for everyone, to protect high-risk populations such as those over 60 and those with comorbidities. 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. Decision makers should take both natural immunity and vaccination status into consideration to obtain a full picture of an individual's immunity profile.”
MedicalXPress reports that in a linked comment in The Lancet, Professor Cheryl Cohen, National Institute for Communicable Diseases, South Africa, who was not involved in the study, wrote: “The high and sustained levels of protection conferred by previous infection against severe disease have important implications for Covid-19 vaccine policy. By September 2021, global SARS-CoV-2 seroprevalence was estimated at 59%, with substantial variation in the proportion of immunity induced by infection or vaccination in different settings. Seroprevalence in Africa was estimated at 87% in December 2021, largely as a result of infection.
“High levels of immunity are an important contributor to the lower levels of severity observed with infection caused by emerging omicron subvariants. 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.”
Study details
Past SARS-CoV-2 infection protection against re-infection: a systematic review and meta-analysis
COVID-19 Forecasting Team
Published in The Lancet on 16 February 2023
Summary
Background
Understanding the level and characteristics of protection from past SARS-CoV-2 infection against subsequent re-infection, symptomatic Covid-19 disease, and severe disease is essential for predicting future potential disease burden, for designing policies that restrict travel or access to venues where there is a high risk of transmission, and for informing choices about when to receive vaccine doses. We aimed to systematically synthesise studies to estimate protection from past infection by variant, and where data allow, by time since infection.
Methods
In this systematic review and meta-analysis, we identified, reviewed, and extracted from the scientific literature retrospective and prospective cohort studies and test-negative case-control studies published from inception up to 31 September 2022, that estimated the reduction in risk of Covid-19 among individuals with a past SARS-CoV-2 infection in comparison to those without a previous infection. We meta-analysed the effectiveness of past infection by outcome (infection, symptomatic disease, and severe disease), variant, and time since infection. We ran a Bayesian meta-regression to estimate the pooled estimates of protection. Risk-of-bias assessment was evaluated using the National Institutes of Health quality-assessment tools.
Findings
We identified a total of 65 studies from 19 different countries. 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. Pooled effectiveness against re-infection by the Omicron BA.1 variant was 45·3% (95% uncertainty interval [UI] 17·3–76·1) and 44·0% (26·5–65·0) against omicron BA.1 symptomatic disease. Mean pooled effectiveness was greater than 78% against severe disease (hospitalisation 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 78·6% (49·8–93·6) at 40 weeks. Protection against re-infection by the Omicron BA.1 variant declined more rapidly and was estimated at 36·1% (24·4–51·3) at 40 weeks. On the other hand, protection against severe disease remained high for all variants, with 90·2% (69·7–97·5) for ancestral, Alpha, and Delta variants, and 88·9% (84·7–90·9) for Omicron BA.1 at 40 weeks.
Interpretation
Protection from past infection against re-infection from pre-Omicron variants was very high and remained high even after 40 weeks. Protection was substantially lower for the omicron BA.1 variant and declined more rapidly over time than protection against previous variants. Protection from severe disease was high for all variants. 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.
Medscape article – COVID Infection Provides Immunity Equal to Vaccination: Study (Open access)
The Lancet Comment – COVID-19 infection, reinfection, and the transition to endemicity (Open access)
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