The Covid pandemic is far from over as daily numbers are still high in many countries, without their epidemic waves declining to low and steady figures that represent endemic prevalence. On 27 June 2021, India transitioned from the epidemic phase to endemic prevalence, sustained for the past 140 days as of November 14.
Are we out of the woods yet? After the first wave abated, we entered a 10-week endemic phase, only to be interrupted by the second wave. The Delta variant of the second wave had far higher transmission efficiency than the first wave variant (Wuhan-G614D). The recent AY.4.2 variant remains below 0.1%, showing low transmission efficiency that cannot overtake Delta transmission. India has thus become the world’s first country to reach endemic prevalence.
Epidemic means daily numbers of Covid cases rise to a crescendo and decline until a steady state with low numbers (endemic prevalence) is reached. The second wave peaked on May 6 (4,14,433 cases) and declined to less than 50,000 cases per day (seven-day moving average) on June 27. After 73 days, on September 8, daily numbers fell below 40,000; after 16 days (September 24) below 30,000; after 14 days (October 8) below 20,000; after 19 days (October 27) below 15,000, sustained for 18 days (November 14).
Transmission dynamics is represented by R, indicating numerical variations of cases on a continuous scale. During the ascending phase of the epidemic, R is greater than 1 and during the descending phase, it is less than 1. During the endemic phase, R is equal to 1 (with minor fluctuations). As the susceptible population pool shrinks, with daily infections and progressive increase in vaccination coverage, case numbers must slowly dwindle. The continuous addition of an immunity-naive population through birth allows continued endemic transmission.
At this historic juncture, we ask two questions: What determined the transition? What changes in strategy should India adopt to mitigate the ill-effects of endemic Covid?
Nearly 85% of Covid infections are asymptomatic; only 15% exhibit symptoms of disease—therefore the reported Covid cases are only a subset of infected people. Epidemic and endemic prevalence refer to disease numbers, irrespective of the magnitude of the invisible backdrop of asymptomatic infections. Although re-infections do occur, they are mostly subclinical without contributing significantly to disease numbers.
Why should Covid epidemic transition to the endemic phase? At any point in time, the number, proportion and distribution of non-immune versus immune people determines the transmission speed R. When the pool of the susceptible is large, R rises above 1 and when the pool of the immune is large, R falls below 1. The end of the epidemic denotes that the herd immunity threshold (HIT) applicable to the particular variant has been reached. HIT is the proportion of immune individuals required for the transition from epidemic to endemic prevalence, when rapid transmission (R>1) and rapid decline (R<1) settle down to slow steady transmission (R=1).The HIT of Wuhan-G614D was a fraction of that of the Delta variant, explaining the sequence of the first wave, endemic phase and the large second wave.
ICMR had conducted periodic surveys of the proportions of population with immunity. The fourth survey in July, after the second wave had ended, showed 67.4% of those above six years of age had antibodies. In the second survey, only 64% of those with previously RT-PCR positive infection had detectable antibodies. Although antibody titres wane below the range of test detection, immunity does not disappear altogether. Therefore, the immune population was much higher than 67.4%, as many among the antibody negative 32.6% would have had previous infection and immunity. Thus, a huge majority had been infected with some virus variant and the HIT of the Delta variant had been reached and surpassed—this is the reason why the epidemic ended. Now the immunity-naive population is not large enough to allow a third wave, unless a variant that defies current levels of immunity in the population emerges. The variant horizon is under constant watch by scientists.
Our Covid vaccination roll-out was aimed at reaching the HIT of the Delta variant for ending the epidemic. Now that we have reached there, vaccination policy has to be revised to achieve two ends: mitigate risks of severe disease during endemic prevalence and reduce the sources of infection.
The risks of serious disease when infected are the same during epidemic and endemic phases for the vulnerable—pregnant women, people aged above 60 or those with comorbidities and conditions affecting the immune system, like those having immune suppressants, organ transplantation, cancers and their treatments, etc. They must be protected by high levels of immunity elicited by two doses during pregnancy the first time and a single booster dose in the next pregnancy, and in all others, a booster dose six months to one year after the second dose. Booster doses save lives and retard further virus transmission.
The sources of virus now are two-fold: all unvaccinated and all with waned immunity after two doses of the vaccine (breakthrough infections) or after past infection (reinfection). Keeping this in mind, the focus of the vaccine roll-out should shift towards achieving source-reduction.
The most important source of the virus, especially as schools reopen, is school children. Their vaccination is an urgent priority during the endemic phase; fast-tracking safety data collection, regulatory assessment and extension of emergency use authorisation for children are urgent needs.
The next priority for vaccination (including boosters) ought to be for those whose occupation brings them in contact with numerous people in social interactions—healthcare, police, religious, education, commerce and sales, transportation, manufacturing, hospitality, etc. Completion of their inoculation can be supervised by their respective administrative officers. An enabling policy revision is needed to accomplish this.
Currently, India is the only country in the world to have reached a sustained endemic state while in other nations, the pandemic is still raging. This is a historic opportunity for us to show the world how to tackle endemic Covid-19.
Dr T Jacob John, Former Professor of Clinical Virology, CMC, Vellore
Dr M S Seshadri, Medical Director, Thirumalai Mission Hospital, Ranipet