Imperatives after India’s September virus peak

After having peaked in the middle of this month, COVID-19 infections could continue till March before turning endemic

India’s COVID-19 epidemic curve appears to have peaked during the middle two weeks, in September 2020, followed by a downtrend since then — if we work backwards for four weeks from September 26, 2020. Let the numbers tell the story.

What the data reveals

The weekly totals of reported new infections in those middle two weeks, namely, September 5 to 11 and September 12 to 18, were 6,37,136 and 6,48,096 cases, respectively. The week before the peak (August 29 to September 4), the total was 5,58,999. And the week after the peak (September 19 to 25), the number was 5,96,096 cases.

During the two peak weeks, the weekly average was 642,616 cases, and in the two flanking weeks, the average was 577,547 cases. The mean daily numbers in the pre-peak week were 79,857; on no day did the number reach 90,000. During the two peak weeks, the mean daily number was 91,801; on 12 days, the number had exceeded 90,000.

In the post-peak week, the mean daily number was 85,156; only on the first day of that week did the day’s number cross 90,000 cases.

As the numbers of documented infections are determined partly by the daily volume of tests, which had declined during the last several days, there cannot be too much reliance on these numbers alone. We need additional supportive evidence.

Lab tests, herd immunity

What is the hidden burden of infections in India? The report of the sero-survey conducted by the Indian Council of Medical Research (ICMR) in May-June and published in September, gave us some surprises. It showed that the number of infections detected by reverse transcription polymerase chain reaction, or RT-PCR testing was a small fraction of the total burden in the community that remained undetected. For every laboratory-diagnosed infection, there were 80 to 100 undocumented infections in the country.

As of September 26, when we analysed the numbers, the cumulative total of laboratory-proven burden of infection for India was 5,990,581, a mere 9,419 short of six million. Using the correction factor of 80-100 proposed by the ICMR, India’s total burden of infection was between 480 million and 600 million.

In India’s population of 1,380 million, the proportion infected — in other words the herd immunity — was in the range of 35% and 43%. Since about 30% herd immunity is sufficient to reach the peak of the epidemic curve, we can be confident that India indeed has reached the peak of the COVID-19 epidemic.

In an epidemic of a directly human-to-human transmitted microbe, graphically represented by the more or less symmetric bell-shaped epidemic curve, about equal numbers of people will be infected before the peak and after the peak. That is during the epidemic phase.

For the pandemic influenza of 2009 which had about the same degree of infectiousness, a proportion of individuals remained uninfected, constituting the pool of susceptible people who sustained its endemic phase. Annual birth cohorts, about 27 million in India, add to the susceptible pool. The true addition will be about 25 million, the rest deducted for premature mortality.

Post-peak scenario

If 35% of the population was infected pre-peak, another 35% will be infected post-peak, for a total of 70% during the epidemic. The residuum of 30% is sufficient to sustain the microbe in the human population, but not as an epidemic but as low numbers, which is the endemic phase of infection. The endemic phase is perpetual, unless interrupted by vaccination. As more people are infected, new cohorts of children replace them to make the numbers up. By simple arithmetic we can foresee some 20-25 million infections annually. The logic is that input and output have to be balanced in any steady state system, endemic prevalence included. In short, we must anticipate 15-18 infections per 1,000 population every year — more in some years and less in others.

When can we expect the epidemic to settle down and become an endemic phase? Our epidemic began in mid-March and peaked after six months, in mid-September. So it is reasonable to assume that the epidemic will continue for a further six months, until mid-March 2021, before it turns endemic.

Vaccination and antibody test

The steady state is over the long term, not the short term. So, unless interfered with using vaccination, we can expect low seasons and high seasons; low years and high years. That is what influenza has taught us. However, the risk of severe disease and death will remain among senior citizens and those with chronic diseases. Vaccination is the ready answer to prevent death in these vulnerable subjects.

How will a vaccine modulate these numbers? As we had said in this daily on August 13, 2020, in an article (Editorial page; https://bit.ly/36f2DaP), “More than a vaccine, it is about vaccination”, a vaccination programme is necessary to protect life and reduce the disease burden. If a vaccine becomes available during the epidemic phase, the epidemic can be cut short quickly.

During the endemic phase, vaccination can protect vulnerable individuals. Protection is important for those who are susceptible to severe disease and death, namely senior citizens and those with comorbidity. Identification of susceptible subjects is possible using an antibody test. Ideally, the immunoglobulins, IgG, IgA and IgM, should be tested, as in Iceland where country-wide screening was done.

If 70% of the population is infected, vaccination is required only in the remaining 414 million people and also the ~7,50,000 new additions per day by birth. The number that needs the antibody tests is about 1,368 million, assuming that nearly 12 million would have had confirmed infection by the time a vaccine is available. Known infected and antibody positive persons need not be vaccinated. Since an antibody test will be cheaper than a vaccination with two doses, funds can be conserved if the need for vaccination is determined first. The government should create facilities for large-scale antibody testing with indigenous production of test reagents.

These steps, large-scale total antibody testing and vaccine delivery for those who are antibody negative will entail expenses that must be accommodated in the budgetary allocation for health. They can be more than recovered by the liberation of the economy from COVID-19 constraints.

Using India’s strength at WHO

We also have a unique opportunity to eradicate COVID-19 altogether if we prepare now for the strategic use of vaccines globally. Judging by the speed with which Phase 3 trials are progressing, we can expect a few vaccines emerging before March 2021. Eradication is a global need, for which India can provide leadership, with Indian officials of influence in the World Health Organization. It must be noted that the chair of the World Health Assembly and the Chief Scientist are both Indians.

T. Jacob John is former Professor and HOD, Clinical Virology Department, CMC Hospital, Vellore, and former President, Indian Academy of Pediatrics. M.S. Seshadri is former Professor and HOD, Clinical Endocrinology Department, CMC Hospital, Vellore, and is currently Medical Director, Thirumalai Mission Hospital, Ranipet, Tamil Nadu