The third wave in India that began in the first week of January 2022, driven primarily by the BA.1 Omicron sub-lineage, peaked at around 3,38,000 new infections on January 21, and came to an end by the first week of March 2022. Except for a short period — mid-June to mid-August — after the third wave, India has not witnessed even an uptick in new infections in 2022. The relative calm in India comes even when new Omicron subvariants and recombinants with ever increasing ability to evade immune responses and greater transmissibility have emerged at regular intervals last year and have led to sudden surge even in hospitalisations and deaths in many countries.
While low levels of testing might wrongly indicate low virus spread in the population, the low test positivity rate and low levels of the virus in wastewater or sewage water surveillance strongly suggest that virus circulation is indeed low in India.
“The hybrid immunity — natural infection and vaccination induced immunity — is the reason that there has not been an uptick in COVID-19 cases in India. An estimated 95% of India’s population above 12 years of age has developed hybrid immunity. India’s situation of hybrid immunity is arguably the best possible protection against SARS-CoV-2,” says Dr. Chandrakant Lahariya, consultant physician and an epidemiologist.
Citing a systematic review published recently in The Lancet, Dr. Giridhara Babu, epidemiologist at the Public Health Foundation of India, Bengaluru, says hybrid immunity from vaccination (both primary, and primary plus booster dose) and previous infection has provided sufficient protection against reinfection for people in India.
“Individuals with hybrid immunity had the highest magnitude and durability of protection [against Omicron variant], and as a result might be able to extend the period before booster vaccinations are needed compared to individuals who have never been infected,” says The Lancet review.
Explaining the absence of surges last year, Dr. Vinod Scaria, a senior scientist at Delhi’s Institute of Genomics and Integrative Biology (CSIR-IGIB), says that though greater immune escape by Omicron subvariants indeed make people susceptible to reinfections, the resulting infection is self-limiting and most often asymptomatic, thus possibly not being tested or recorded.
What makes the absence of new infections surging after the third wave ended is the relative ease with which the Omicron subvariant BA.2.75, which was first detected in India, caused cases to spike between mid-June and mid-August last year; the subvariants and recombinants of Omicron that emerged after BA.2.75 have even greater immune escape and transmissibility but have not caused any spike in cases.
“Epidemiologically, only those people who were not exposed to Omicron till then developed symptomatic infection by BA.2.75,” says Dr. Lahariya. “In the third wave, 60-65% of the population was exposed to Omicron. This means 35-40% of the population still remained susceptible to Omicron.” But there have numerous recorded instances in other countries where people have been reinfected by more than one Omicron subvariant.
“After high vaccination rates, the rise in cases due to BA.2.75 has been ironically advantageous; despite escaping the immune response, it has managed to infect most people without causing severe disease. In retrospect, the population in India may have had a better immune profile against XBB.1.5 due to hybrid immunity,” Dr. Babu says in an email. Evidence from the high-risk populations suggests that vaccination and previous infection were effective against Omicron infection by 96% and 72%, respectively.”
Explaining the low level of daily fresh cases seen for most part of 2022, Dr. Scaria says, “If everyone were vaccinated or infected at a short interval, almost all would lose the antibodies at almost the same time making them possibly susceptible. If a constant but low percentage of people are being infected, it would mean that a majority of people have varying levels of immunity. So only a subset of individuals is susceptible at any point in time.”
Besides hybrid immunity, Dr. Lahariya cites another possible reason for the low daily fresh cases due to Omicron subvariants and recombinants. People who have been exposed to Omicron BA.1 and/or BA.2.75 may have a specific type of antibodies against other Omicron subvariants. Now, if they are infected again, the potential of the virus multiplying in their nose and throat going further deep down in lungs is low, he says. That has two implications — people will not develop severe disease, and the amount of virus they pass on will be less, thus reducing the risk to others. But he cautions that this scenario is restricted to Omicron and may change when a new variant emerges.
In an endemic stage of constant circulation of virus, we expect a low level of positivity, as we see now, says Dr. Scaria.
While Dr. Lahariya says that COVID-19 waves in India in the future will be small, mostly unpredictable and clinically not relevant. “In the absence of emergence of a new variant of concern, it might be impossible to differentiate between seasonal flu and COVID-19,” he says.
Despite the low daily fresh cases now being reported, Dr. Babu is quick to caution against completely dropping our guard against the virus. “It is a dangerous and premature conclusion to say that COVID-19 is no longer a public health issue in India based on a trend seen since March 2022. The virus is still present in the population of many countries. The more time the virus circulates, the higher the chance for the virus to get mutated. The emergence of new variants of the virus can also pose a significant threat to the population. It is crucial to remain vigilant and remain proactive rather than proclaim victory prematurely,” says Dr. Babu.