Study finds ‘hybrid immunity’ most effective against severe Covid: the findings, implications explained

However, with the faster-spreading omicron variants leading to more infections and consequently more people developing this hybrid immunity, the study suggested that booster doses may be delayed.

“We already know that hybrid immunity provides the best protection – an infection after vaccination acts like a booster. Getting a natural infection – best after vaccination so that the risk of death is lower – provides better protection than vaccines alone because it prepares the body against the entire virus, rather than say just the spike protein,” said Dr Pragya Sharma, director, Andaman & Nicobar Islands Institute of Medical Sciences.

What did the study find?

Protection against severe disease and hospitalisations from a Sars-CoV-2 infection alone was found to be 82.5% at three months after the last shot or infection. This protection stood at 74.6% at 12 months and 71.6% at 15 months. Protection against reinfection declined faster, standing at 65.2% at three months and dropping to 24.7% at 12 months and 15.5% at 15 months.

In comparison, hybrid immunity with just the primary vaccine doses was found to be 96% at three months and 97.4% at 12 months. The same can offer 69% protection against reinfection at three months, dropping to 41.8% at 12 months.

The effectiveness of hybrid immunity gained from infection coupled with the primary as well as a booster dose stood at 97.2% at three months and 95.3% at six months. The same immunity was found to be 68.6% effective at three months and 46.5% at six months.

Implications of the findings

The study said, “These results provide information that can be used to tailor guidance on the number and timing of SARS-CoV-2 vaccinations.”

It said that in regions with high Sars-CoV-2 sero-prevalence, the primary vaccination – focused mainly on those at the highest risk of severe disease such as the old or co-morbid – can offer high protection against severe disease and hospitalisation for at least one year.

As per the World Health Organisation, the global sero-prevalence – presence of antibodies against Sars-CoV-2 whether because of infection or vaccination – stood at 67% as of October 2021. Two-thirds of Indians had these antibodies as early as June-July 2021 after the delta variant wave earlier that year.

The other key recommendation is to roll out booster vaccine drives whenever an increase in the number of infections is expected.