That natural immunity following a virus infection stays robust and lasts longer is already well known. People infected with the 2002 Severe Acute Respiratory Syndrome (SARS) and the Middle East Respiratory Syndrome (MERS) have been shown to have strong immunity for up to three years, while the immunity lasts for life after a chickenpox infection. Now, a vaccine effectiveness study undertaken in Israel has shown that natural infection confers stronger immunity against the SARS-CoV-2 virus than even full vaccination. They found that people previously infected with the SARS-CoV-2 virus had better immunity and had reduced risk of reinfection, symptomatic disease and hospitalisation caused by the highly transmissible Delta variant than uninfected people who were fully immunised with the Pfizer vaccine.
The results have been posted on medRxiv, a preprint server. Preprints are yet to be peer-reviewed and published in scientific journals.
Since Israel was the first country to aggressively vaccinate a large percentage of the population with the Pfizer vaccine, the researchers were able to compare over 16,000 people who were previously infected but not vaccinated with an equal number of people who had not been naturally infected but fully vaccinated.
During the follow-up, it became clear that even full vaccination with the Pfizer vaccine did not confer immunity that was superior to the one acquired through natural infection. There were 238 instances of breakthrough infections in the fully vaccinated compared with just 19 reinfections among those previously infected but not vaccinated. The infection or vaccination occurred during January and February this year. After adjusting for comorbidities, it was found that the risk of breakthrough infections was 13-fold higher than reinfection among the naturally infected group. Differences in protection were seen in symptomatic diseases too. At 191, the number of people with symptomatic disease in the fully vaccinated group was higher than in the previously infected group, which remained at eight. After adjusting for comorbidities, there was a 27-fold risk for symptomatic breakthrough infection compared with symptomatic reinfection.
But waning of natural immunity against the Delta variant was seen when infections that had occurred anytime between March 2020 and February 2021 were compared with vaccination during January-February 2021. The risk of breakthrough infections and symptomatic disease in the fully vaccinated was nearly six-fold and over 7.1% higher respectively than in those previously infected. Risk of hospitalisation was also higher among the vaccinated.
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This study does not tell if the level and duration of protection varies depending on the severity of the disease, and whether asymptomatic infection confers the same protection as those with the disease. Since the correlates of protection are not yet known, it is unknown if the broad immune response from natural infection might be proving to be superior to antibodies generated in response to spike proteins in the case of vaccines.
Though this is the largest real-world study evaluating the effectiveness of the Pfizer vaccine in preventing infections compared with natural immunity, it is only an observational study. Testing for the infection was not undertaken, thereby underestimating asymptomatic infections. Also, the number of events — infections or symptomatic disease — in all groups were fewer.
More such studies involving Pfizer and other vaccines, carried out over a longer time period, are needed to fully understand the level and duration of protection conferred by COVID-19 vaccines. One encouraging finding is the absence of death among the vaccinated, a clear signal that the vaccine offers formidable protection against serious disease. Hence, natural immunity, even if found to be superior and long-lasting than vaccine-induced protection, is not what one should opt for.
Since January 2020, there have been 4.5 million COVID-19 deaths recorded globally, a vast majority of which could have been prevented. A sizable number of deaths have been among the healthy and those younger than 60 years, which flies in the face of a section of scientists that came up with the Great Barrington Declaration before vaccines became available.
Vaccination will always remain a safe and sure way to remain protected against severe COVID-19 disease and death, even if it means the protection is not highly robust or long-lasting.
prasad.ravindranath@thehindu.co.in
