A vaccine strategy for endemic Covid in India
In the last 12 weeks, since 1 July 2021, the daily new cases of Covid in India (seven-day moving average) has remained consistently below 50,000 and is beginning to decline slowly. Test positivity rate has consistently remained below 2.5% (covid19India.org). We can heave a sigh of relief as these numbers represent an ‘endemic’ phase nationally, except in Kerala, Mizoram and Meghalaya, where the second wave has not yet ended. In Kerala a downward trend has begun in the second week of September.
This good news is a signal for the government to accelerate economic recovery, open up educational institutions and enhance livelihood options for citizens. Such relaxations are of fundamental importance for general social well-being. According to WHO, health is not merely the absence of disease, but also a state of complete physical, mental and social well-being.
Endemic state denotes stable numbers of disease cases with only minor fluctuations. When an epidemic comes in successive waves, there may be endemic intervals in between, as we saw during the eight weeks between the first and second waves. The present endemic phase has lasted 12 weeks and is likely to continue, unless a virus variant that has even higher transmission efficiency than the current Delta variant emerges.
Two factors are necessary for waves—the persistence of the virus in the community (endemicity) and a pool of non-immune, susceptible persons, of sufficient size to support an outbreak.
During the endemic period, there will be some adults who had previously escaped infection, along with some with waned immunity. Their cumulative number is not sufficient to support a wave as of now. The pool size increases with new births—in India ~70,000 per day. Therefore, daily that many (minus mortality) are added to each sliding annual age cohort, from childhood to adults.
Influenza, measles and chickenpox are endemic in India, with periodic outbreaks as waves—influenza annually, measles once in two-three years and chickenpox once in five-ten years. Based on these examples we must anticipate waves for Covid too, but the periodicity (duration of inter-wave endemicity) has to be observed rather than guessed.
Periodicity is modulated by the transmission efficiency of the virus, measured as ‘basic reproduction number’ Ro, which in turn determines the size of the susceptible pool necessary for a wave; lower the Ro, larger the required pool, and, higher the Ro, smaller the required pool size. India’s first Covid wave virus, the Wuhan-D614G variant, had low transmission efficiency (Ro of 1.5–2). The post-first wave endemic phase was cut short by the Delta variant, with four times higher Ro, namely 6–8.
Both waves together have infected a majority of the population, resulting in high prevalence of immune individuals—the very reason the second wave abated. The latest nationwide sero-prevalence of 67% across urban and rural populations confirms this conclusion. Antibody prevalence underestimates true prevalence of functional immunity. Using the correction factor reported by ICMR, about 85-90% of the population are probably immune. The size of the remaining pool of susceptible population is insufficient to sustain a flare up as of now. Over time new births will add up to the needed threshold for a wave.
Alternatively, a variant with higher Ro than that of Delta could emerge. However, for now, it is important to realise that even during endemic transmission, non-immune persons could acquire infection anytime, anywhere. Senior citizens and those with comorbidities or immunosuppression carry the same level of risk for severe disease, need for hospitalisation and mortality, just as during a wave.
The vaccination drive began in India on 16 January 2021; currently ~16% of the population have received two doses—average 1.8% per month (covid19India.org). The purpose of universal vaccination coverage was to prevent a new wave but the second wave began in March, peaked in May and abated by the end of June. A new wave seems improbable despite low vaccination coverage—therefore vaccination can be redesigned for specific objectives that are the need of the hour.
One objective is to save lives and avoid hospitalisations. To be medically meaningful, all individuals with the above-mentioned risk factors who are still unvaccinated, should be individually identified and vaccinated. Scientific information warns that even in the vaccinated, antibody titres decline faster and breakthrough infections are more frequent among such individuals; hence provision should be made for booster doses six months after the second dose. The six months interval will help in conserving available vaccines to give the second dose to those who have received the first, and the first dose to the unreached. There is hope that from October, India will have enough vaccines for domestic need and to fulfil international obligations.
Other priority objectives are to safely reopen educational institutions, accelerate economic recovery and enhance livelihood/employment options for citizens.
Reopening schools and colleges has started in some states. All above age 18 are currently eligible for vaccination. Covid in children has a milder course overall; serious problems like Multi-System-Inflammatory Syndrome are very rare and readily manageable. Vaccinating children below 18 can wait till vaccine safety is documented, but we need not wait for it to open schools. However, we must create a protective mantle around children by vaccinating every eligible adult at home and all workers in all schools—teaching, administrative, service and transport.
For accelerating economic recovery, manufacturing and businesses need to reopen and inter-state transport services restored. All such establishments should be supplied with sufficient vaccines for all staff, with instructions, through local health authorities. This approach should cover all transport workers, and those in tourism and hospitality industries—hotels and restaurants.
We anticipate that migration of labourers (inter-state and rural-urban) and workers in the unorganised sector may be overlooked. A systematic and innovative process should be designed and implemented, ensuring the entire workforce in the country (skilled and unskilled, in unorganised sectors, household workers, self-employed) is vaccinated.
This strategy will eventually result in universal vaccination, but the responsibility for targeted vaccination to achieve the four priority objectives will require wider participation of government departments and district and local administrations.
Step one is to evolve policy; then the public should be informed. Any vaccine reluctance should be overcome through education and nudging rather than mandates. Employers can rightfully consider vaccination a prerequisite for employment—in the interests of co-workers, uninterrupted productivity and employment.
Dr T Jacob John
Former Professor of Clinical Virology,
CMC, Vellore
(tjacobjohn@yahoo.co.in)
Dr M S Seshadri
Medical Director,
Thirumalai Mission Hospital, Ranipet
(mandalam.seshadri@gmail.com)