A previous infection with COVID-19 can provide an 88 per cent protection against hospitalization or death in the case of reinfection even 10 months later, according to a new study.
A looked at more than 65 studies from across 19 countries and helped researchers put together the clearest picture yet of immunity derived from a previous COVID-19 infection, and how it wanes over time.
When a person recovers from a virus, their body will have learned from the experience and retain some measure of protection against the virus in the future, something called natural immunity.
However, the effectiveness of natural immunity varies depending on the virus, and with reinfections now a common phenomenon while COVID-19 continuously morphs into new variants, it has been difficult to pin down what level of protection is retained after recovering from COVID-19.
According to researchers of this new study, a previous infection can provide significant protection against severe disease and death in case of reinfection with numerous COVID-19 variants. It noted though that protection derived from a pre-Omicron variant is much less effective against reinfection with an Omicron variant.
Researchers also found that while protection derived from past infection with a pre-Omicron variant was very effective at stopping re-infection with a non-Omicron variant even after 40 weeks, protection derived from past infection with the Omicron BA.1 variant was lower and declined faster over time.
Researchers stressed that the public shouldn’t rely solely on natural immunity or hold off on getting vaccinated simply because they’ve had COVID-19 previously, emphasizing that vaccination is the best way to achieve protection against COVID-19 and minimize the creation of new variants.
“Vaccines continue to be important for everyone in order to protect high-risk populations such as those who are over 60 years of age and those with comorbidities,†said Dr. Caroline Stein, of the Institute for Health Metrics and Evaluation (IHME) at the University of Washington’s School of Medicine, in a press release.
“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.â€
The analysis found that the duration and strength of natural immunity provided by previous infection was at least on par with that provided by two doses of Moderna or the Pfizer-BioNTech vaccines.
The review did not look at studies that examined hybrid immunity — those who had received some level of vaccination and also had previously contracted COVID-19 — nor did it include infections with Omicron XXB or its related sublineages.
“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,†Dr. Stephen Lim, lead author of the study from IHME at the University of Washington’s School of Medicine, said in the release.
Not only does a person have to contract COVID-19 to get natural immunity, risking severe illness, long COVID or even death, but as this study suggests, protection from a previous infection may be ineffective against newer variants.
When looking at protection against re-infection, ancestral, Alpha, Beta and Delta had a much higher pooled effectiveness estimate than Omicron BA.1. For instance, while Delta provided a pooled protection of 82 per cent against general reinfection, Omicron BA.1 provided only around 45 per cent protection against general reinfection.
But this was reinfection regardless of specific variant.
When looking specifically at trying to prevent reinfection with Omicron, earlier variants provided less robust protection on that front, according to the review.
Around 21 studies included in the review demonstrated that natural immunity derived from infection with a pre-Omicron variant quickly dwindled in effectiveness when it came to protecting against reinfection from the Omicron BA.1 variant.
One month after infection with a pre-Omicron variant, the protection against reinfection with Omicron BA.1 fell to 74 per cent. Ten months later, there was only a 36 per cent protection against reinfection with Omicron BA.1, the review found.
Protection against reinfection with earlier variants was much longer lasting.
“The weaker cross-variant immunity with the Omicron variant and its sub-lineages reflects the mutations they have that make them escape built-up immunity more easily than other variants,†Dr. Hasan Nassereldine, an IHME co-author,
“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 per cent of the global population between November 2021 and June 2022. 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.â€
Protection against symptomatic disease was also much lower when it came to previous infection with Omicron BA.1, according to five studies covered in the review. While a previous infection with Delta, Beta, Alpha or the original strain of COVID-19 provided around 82 to 87 per cent protection against symptomatic disease in the case of reinfection, a previous infection of Omicron BA.1 only provided around 44 per cent protection against symptomatic disease in the event of reinfection.
Six studies covered in the review focused on reinfection with the specific Omicron sublineages of BA.2 and BA.4/BA.5, and found that although there was significantly reduced protection from previous infection when that infection was pre-Omicron, previous infection with an Omicron variant provided more protection.
“When the past infection was Omicron, protection was maintained at a higher level, although less so for BA.4 and BA.5, confirming the greater immune escape associated with this sublineage,†the study stated.
Researchers looked at all studies published on protection related to past COVID-19 infection up to the end of September 2022, a total of 65 studies. Thirty of the studies included information on the time since infection, and 18 of those explicitly measured protection through the time after infection.
As time passes, the immunity granted by a previous case of COVID-19 slowly fades, the review found.
The 21 studies which looked at infections after Omicron variants emerged found that a pre-Omicron variant provided 85 per cent protection against reinfection with another pre-Omicron variant at one month after infection. This fell to around 79 per cent at 10 months after infection.
Researchers found that five studies which focused on severe outcomes — hospitalization and death — found protection against these was still high 10 months after infection, with a 90 per cent protection against severe disease conferred by previous infection the original strain of COVID-19, Alpha or Delta, and an 88 per cent protection against severe disease conferred by previous infection with Omicron BA.1.
The review was limited by the studies available to them, with researchers noting that there were far more studies relating to natural immunity’s protection against reinfection than there were studies which focused on natural immunity’s protection against severe disease in the event of reinfection.
The studies assessed also used varying ways of defining protection levels, from T-cell counts to measuring antibody levels, and varying ways of ascertaining the past infection status of participants, meaning there could be incomplete data in some areas.
While researchers cautioned that their findings should not be used to discourage vaccination, they suggested the protection provided by a previous infection could be taken into account when crafting policies which require vaccination for certain activities such as travel.
“Decision makers should take both natural immunity and vaccination status into consideration to obtain a full picture of an individual’s immunity profile,†Stein said.
A previous version of this story stated that infections from older variants provide less protection against reinfection compared to newer variants like Omicron. It has been updated to reflect that infections from older variants provide less protection against reinfection with newer variants like Omicron.
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