Comment

The time for — not mass but targeted vaccination

While the COVID-19 pandemic rages on in the United States and the United Kingdom, the scene is very different in India with the peak having passed in mid-September 2020, and there now being a current sustained down-trend. In the last four weeks (December 24-January 21), the reported average daily infections were 22,263; 15,805; 15,541 and 13,956. The epidemic phase is coming to an end, and the endemic phase, imminent.

The dynamics

A person-to-person transmitted infection is endemic, when, on the average, each infection reproduces another, for ‘reproduction number’ R = 1. When R = > 1, daily infections increase, signalling rapid spread — the ascending limb of an epidemic curve. Post-peak, the curve declines, with R = <1. Finally, when R settles down to ~1, infection is endemic — daily number of new infections remains stable with minor fluctuations.

The transmission dynamics of ‘endemicity’ is interesting. During this steady state, the population is a pool of individuals wherein one infected person infects only one other person, the number of daily infections remaining constant. By our estimates of the daily new births in India, by age six, around 54,000 survive over time. Therefore, we add 54,000 susceptible subjects daily to the pool, disturbing this equilibrium. If, in spite of this addition, daily infections remain constant, it implies that new infections equal the susceptible subjects entering the pool. In India, during the COVID-19 endemic phase, average daily infections will be ~54,000 with minor fluctuations.

Only a fraction of total infections is detected by current testing strategy. To get the true numbers, we need to use arithmetic and the principle of triangulation.

The reported numbers can be used to compute the true number. During January 15-21, the reported daily number averaged 13,956. The currently popular rapid antigen test detects only 50% of infections; so, the actual number of new infections is double that number, about 28,000.

Nowadays, we mostly test symptomatic individuals, who account for only about half of total infections. Hence, the daily new infections may be 56,000, only 2,000 more than 54,000 — India is very close to reaching endemic state.

Can we corroborate this with the most unequivocal numbers that we have, namely mortality statistics?

The average daily COVID-19 deaths were 163 last week. As mortality certification is only 20% in India, the estimated number of daily COVID-19 deaths (highest estimate) may be five-fold — 815. Our COVID-19 case-fatality is 1%, occurring approximately four weeks from the initial symptoms. Therefore, the number of infections four weeks ago (third week of December) was about 81,500. The epidemic, as per the mortality graph, is waning with a halving time of four weeks. By the third week of January, the daily infections would have declined from 81,500 to 40,750. In short, by the third week of January 2021, India could have already entered endemic phase.

The shape of the epidemic curve in India, a large monophasic wave, negates urban-rural delay in the spread of the novel coronavirus infection; presumably because of the ‘leaky lock-down’ — even the first sero-survey showed considerable rural spread. Therefore, another wave is unlikely.

When the epidemic enters the endemic phase, herd immunity threshold (for COVID-19 ~60% of population infected) is reached. Now is a critical moment in COVID-19 epidemiology — to design appropriate vaccination strategy for the current endemic phase.

Action after a missed chance

In Europe, the U.S. and Brazil, where there is on-going rapid epidemic spread, the appropriate strategic objective is mass vaccination to curtail spread. This window of opportunity to curtail the epidemic was missed in India — the emergency-use authorisation of vaccines was too late.

In the endemic phase, which India seems to be entering, the strategy should be carefully crafted to eliminate COVID-19 deaths and to eradicate the viral infection altogether from the country. Now is the time for targeted, instead of mass vaccination.

Vaccinating health-care staff is a humanitarian and public health imperative; we need a well-protected workforce to deliver care confidently to vulnerable patients without unwittingly transmitting the virus. The public health system need not be taxed — when the vaccine is supplied to health-care institutions, staff manage the rest.

Young people have gradually resumed normal day-to-day activities, transport services restored, educational institutions opening up, and movie theatres permitted to operate. The 60% of the population which got infected for reaching herd immunity threshold are predominantly healthy youngsters. The elderly and the vulnerable with comorbidity who remained cocooned are at risk of infection, severe disease and mortality.

What the priority should be

India’s first priority should be targeted vaccination to prevent deaths. Citizens above 55 years and those below 55 with diabetes, hypertension, obesity, chronic lung, heart and kidney diseases should be vaccinated first — through local governments (corporations, municipalities and panchayats), utilising already available data in the Aadhaar database. Enumeration, registration and vaccination by appointments need not be delayed any further.

As schools reopen, children could acquire infection at school and transmit it to parents and grandparents. Priority vaccination of teachers and ancillary staff would be an essential strategy, along with all protective measures — masks, physical distancing, hand hygiene — until vaccination is approved in children.

An opportune priority is to design an innovative approach for the elimination of the coronavirus infection from the country, in our self-interest and to encourage the resolve for global eradication. Now is the time to lay its foundation; delays may render the disease non-eradicable for two biological reasons.

Mutations, animal reservoirs

The virus is mutating and new ‘variants’ are emerging. If new ‘strains’ emerge, the present vaccines may not be protective and new vaccines may have to be designed. Eradication must pre-empt such eventuality.

Second, now the virus transmission is only human to human, there is no animal reservoir. Animals of the feline (cat, tiger, lion, leopard) and canine (dogs, raccoons) families are vulnerable to human-animal transmission, but not vice-versa. Mustelidae (mink, weasels, ferrets and badgers) and Cervidae (farmed deer) are vulnerable and can transmit the virus to humans. They may form animal reservoirs, rendering eradication impossible.

Before these risks become real, COVID-19 must be eradicated.

Dr. M.S. Seshadri is Medical Director and consultant Physician, Thirumalai Mission Hospital, Ranipet, Vellore, Tamil Nadu. Dr. T. Jacob John is former Professor of Clinical Virology, CMC Hospital Vellore

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Printable version | Feb 28, 2021 4:11:03 AM | https://www.thehindu.com/opinion/op-ed/the-time-for-not-mass-but-targeted-vaccination/article33662289.ece

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