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Second thoughts on Covishield gap? Let’s get in the first jabs first

India’s Covid-19 vaccination drive has never had a dull moment. There is a new high-decibel discourse that India should reduce the dose interval between two shots of Covishield vaccine. So, should it? Maybe in the months ahead when the high-risk population has received at least one jab and the vaccine supply becomes assured, but not now.

Don’t ape: By the time, the UK reduced Covishield gap to 8 weeks, nearly 80% of the adult population had received at least one dose. This is not the case here

Before coming to the reasoning for that, let’s look at what has given a boost to the discourse to urgently reduce the dose. First, is the inevitable comparison with the UK’s vaccine program since the two countries share the most commonly used Covid-19 vaccine. Yet, India started with 4-6 weeks in two shots of Covishield; while the UK with 12 weeks of gap. Four months later, when India switched to 12-16 weeks of gap, within days, the UK reduced the dose interval for those older than 50 to 8 weeks, citing the emergence of the Delta variant, which has higher transmissibility, immune escape and causes breakthrough infections. Delta is now the most circulating variant in both the UK and India.

Second is the data from Public Health England (PHE) on real-life vaccine effectiveness which indicated that a single dose of Oxford/AstraZeneca vaccine had shown 30% to 33% protection against symptomatic infections caused by the Delta variant. A few lab-based studies from India and other parts of the world have also reported that many licensed vaccines produced lower levels of neutralising antibodies against the Delta variant (However, all currently licensed vaccines are protective against circulating variants of concern).

But what’s important to note is that the PHE data also reported that even a single dose of the Oxford/AstraZeneca vaccine has 71% effectiveness against severe disease and hospitalisation in people infected with the Delta variant. This single-dose effectiveness is far superior to the benchmark of full schedule vaccine efficacy of 50% set for licensing of any Covid-19 vaccine, against the same end outcome.

Even in India, vaccine effectiveness studies from Chandigarh and Vellore have shown that a single or two doses of Covishield provide nearly similar levels of protection, be it the Delta or Alpha variant. The Delta variant is considered to cause breakthrough infections (people vaccinated getting infected) but two unpublished studies from India have observed that the rate of breakthrough infection continues to be low (1.5% to 5%) and the rate is not very different with single or both doses.

In India, an early second dose does not seem to offer any advantage, at least as of now. Technically, in the early stage of the vaccination drive, the core objective must be to reduce morbidity and mortality reduction. The best approach for this is to deliver at least one shot. At a later stage, once coverage with one dose is high, the focus shifts to reducing transmission. At this stage, the focus has to be full immunisation coverage. With only 15.5% of the total Indian population receiving any dose, India is in early stage Covid-19 vaccination. In contrast, by the time the UK reduced the interval to 8 weeks, nearly 80% of the adult population had received at least one dose. Understandably, by then its focus shifted on achieving full coverage at the earliest possible. Clearly, the cases of UK and India are non-comparable.

It is also not necessary that every country follow the same vaccination schedule. The vaccine dose interval is determined by the combination of scientific evidence, local context and other operational considerations such as availability of vaccines. For example, childhood vaccines in India are administered at the interval of 4 weeks. In many developed countries and even in the private sector in India, the same childhood vaccines are recommended at a gap of 8 weeks or even longer. Each of those approaches serves different purposes.
There is another epidemiological difference. India’s Covid-19 vaccination drive is being scaled up after the ferocious second wave. A large proportion of the population is likely to have developed asymptomatic infection and will now receive a vaccine. A recent Indian study has noted that people with prior infection produce higher levels of antibodies after vaccination. Therefore, it is likely that even a single shot (natural infection and then one vaccine shot) works better.

Vaccination strategies have to be dynamic and evidence informed. To guide decision making, Indian government should use the available data and conduct additional research studies on various aspects of vaccine intervals and effectiveness. Alongside, the available national and global scientific evidence should be used for mathematical modelling to compare the impact of different implementation strategies including dose interval and to arrive at decisions rather than knee-jerk responses. For citizens, there is no reason to be concerned. Just follow the currently recommended dose-interval to get your second shot.



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Views expressed above are the author’s own.



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