Interval between Covishield vaccine doses extended to 8 weeks | Know why

After studying emerging international scientific data, Ministry of Health and Family Welfare  has decided to extend the time-gap between the first and second doses of Covishield to eight weeks. People in India are given Covishield, Serum Institute of India’s version of AZD1222, the vaccine developed by AstraZeneca and University of Oxford and Bharat Biotech’s Covaxin.

This government directive was reportedly based on findings from AZD1222 international trials which found that a vaccinated person’s “protection will be enhanced” if the second dose is given between 6-to-8 weeks and not later. But even as global trials of AZD122 suggested that limiting the duration between the vaccine shots to 12 weeks hiked up the vaccine’s efficacy,  “interim findings” reported on Monday (March 22) that trials in the US, Peru and Chile showed that the vaccine had 79% efficacy, even when the second dose was given four weeks after the first dose.

So, what prompted the government to increase the interval between two dosages of Covishield vaccine?

Two expert groups – The National Technical Advisory Group on Immunisation (NTAGI) and National Expert Group on Vaccine Administration for Covid-19 (NEGVAC), had made this recommendation to the government after examining all the available scientific evidence from global clinical trials.

What did data from these clinical trials show?

Data from global trials of AZD1222 in other countries, showed that the efficacy or the vaccine’s ability to reduce COVID-19 symptoms in those inoculated, was higher when the second dose was given more than six weeks after the first.

Also read: COVID 2.0: Can the country cope with another lockdown?

In a February study, which had analysed COVID-19 cases in participants of phase 3 clinical trial in UK, Brazil and South Africa, AZD1222’s efficacy was around 54.9% when the second dose was administered less than six weeks after the first dose. The efficacy went up to  59.9% when the second dose was given with a 6 to 8 weeks window, 63.7% when the second dose was given between 9 to 11 weeks, and 82.4% when the dosing interval stretched to 12 weeks or more.

But, here’s a rider, this study submitted to The Lancet in February, has not yet been peer-reviewed.

Latest data from interim findings show efficacy for 4-week dosing interval

However,  interim results from phase 3 clinical trials of the Oxford University and AstraZeneca vaccine, which was conducted on 32,000 participants across the US, Chile and Peru showed that the vaccine had a 79% efficacy when the interval between doses was four weeks. The efficacy found in these trials is much higher than its efficacy in trials conducted in countries like the UK and Brazil.

India limited the interval between the doses to 8 weeks. Why?

Health government officials were not convinced that there was enough data to prove that the vaccine will further improve one’s immunity against the coronavirus if the dosage interval was increased beyond eight weeks. Dr N K Arora of NTAGI told Indian Express that they had examined every bit of data that was available and found “no good scientific evidence” to prove that efficacy will improve if the vaccine was given beyond eight weeks. Moreover, health experts felt that India is in a unique position of being “vaccine sufficient” unlike other countries which do not have enough vaccines.

Also read: COVID vaccination drive on fast track, India quickest to reach 40-lakh mark

Secondly, health experts argued that India, which is in the throes of a spike in COVID-19 cases, could not risk infections in the interim period, if they increased the interval between the two dosages. According to Arora, extending the time gap between the two dosages can lead to “dreadful infection” between the first and second dose. If the second dose is given after a 12 week gap, there are chances the person may catch the COVID-19 infection in between the shots. Moreover, extending the dosing interval does not necessarily hike up  antibodies and automatically translate to more protection from the coronavirus.

Will extending the time between the two dosages impact India’s ongoing vaccination campaign in any way?

One, a larger number of people can now get their first dose, including all the elderly. Secondly, there is a lot more flexibility for people – you can get the vaccine anytime between 28 to 56 days.