What does this mean for a “third wave” in India?
A third wave in India is an unlikely scenario with these high levels of antibodies, and vaccination levels continuing to rise.
It’s now recognised those who become naturally infected with COVID and recover before vaccination develop better immunity than those who only have antibodies from vaccination. This is referred to as “hybrid immunity” – those with previous SARS-CoV-2 infection mount unusually potent immune responses to the COVID vaccines.
The Centers for Disease Control in the US notes that both the fully vaccinated individuals and previously infected groups have a low risk of subsequent infection for at least 6 months.
Results of the most recent national serosurvey in India reflect the seroprevalence during the third week of June 2021; the Delta-led second wave had bottomed out at that time. Though about 30% of the population remained susceptible, subsequent serosurveys and an absence of any post-festival surge confirm continuing high levels of protection.
“Patchwork vaccination” areas, where there are pockets of low coverage of vaccination among areas with high levels of coverage, run the risk of small outbreaks, but are unlikely to be large enough to be of any major epidemiological concern.
With high seropositivity among adults, many of the new cases can now be expected among children, particularly with the reopening of educational institutions. But high levels of immunisation among teachers (upwards of 90%) and the emerging evidence that reopening schools has not been associated with significant increases in community transmission, are reassuring.
The WHO’s chief scientist said in late August that India seems to be “entering some stage of endemicity”. Endemic refers to the constant presence or usual prevalence of a disease in a population within a geographic area, where disease spread and rates are predictable.
Could a new variant, such as the Delta Plus subvariant first detected in India in April 2021 threaten the current relative stability? While it has been said it might be about 10–15% more transmissible than the Delta variant, the evidence from Europe suggests it has not yet been able to establish any dominance over Delta.