Here's why interval between Covishield vaccine doses raised to eight weeks

Earlier, the interval between the two doses of the Covishield vaccine was four-six weeks

The Centre on Monday asked States and union territories to increase the interval between two doses of the Covishield vaccine, citing scientific evidence of enhanced protection if the second dose is administered between six to eight weeks.

Earlier, the interval between the two doses of the Covishield vaccine was four to six weeks.

The government made it clear that there has been no change in the interval between two doses of Covaxin, the indigenously-developed vaccine by the National Institute of Virology and Bharat Biotech.

The interval between two doses of a specific Covid-19 vaccine i.e. Covishield has been revisited by the National Technical Advisory Group on Immunization (NTAGI) and subsequently by the National Expert Group on Vaccine Administration for Covid-19 (NEGVAC) in its 20th meeting.

Also read: Delay second dose of Covishield for better protection: Centre

Keeping the existing scientific evidence in view, it appears that protection is enhanced if the second dose of Covishield is administered between six to eight weeks, but not later than the stipulated period of eight weeks.

Covishield is Serum Institute of India’s version of the coronavirus vaccine developed by AstraZeneca in collaboration with the University of Oxford.

AstraZeneca's trials in the United States, Chile and Peru showed the vaccine was 79 per cent effective at preventing symptomatic Covid-19 infection, enhancing confidence in the vaccine after confusion over its efficacy data, dosing regimen and possible side-effects.

According to data from trials of the vaccine in other countries, the efficacy of the vaccine increased when the second dose was given more than six weeks after the first. Efficacy in this case is the vaccine’s ability to bring down cases of symptomatic Covid-19 in those inoculated, compared with those who are not.

Read more: US health officials question results from AstraZeneca’s Covid-19 vaccine trial hours after they’re released

According to an Indian Express report, the vaccine's efficacy was around 54.9 per cent when the second dose was given less than six weeks after the first dose, as per a February study analysing Covid-19 cases in phase 3 clinical trial participants across the UK, Brazil and South Africa. The efficacy increased to 59.9 per cent when the second dose was given 6-8 weeks after the first dose, 63.7 per cent when the second dose was at 9-11 weeks, and 82.4 per cent when the dosing interval stretched to 12 weeks or more. This study, which was submitted to The Lancet in February, has not yet been peer-reviewed.

But, the US health department has expressed its doubt over the AstraZeneca vaccine's efficacy.

The National Institute of Allergy and Infectious Diseases in the US said that the data and safety monitoring board, an independent panel of medical experts under the National Institutes of Health that has been helping to oversee AstraZeneca’s US trial, had notified government agencies and AstraZeneca late Monday that it was “concerned” by information the drugmaker had released that morning.

Also read: White House says will integrate AstraZeneca Covid-19 vaccine to US distribution if approved

The institute urged AstraZeneca to work with the monitoring board “to review the efficacy data and ensure the most accurate, up-to-date efficacy data be made public as quickly as possible.”

Regulators in Europe initiated a safety review of AstraZeneca’s vaccine earlier this month after a small number of people who had recently been inoculated developed blood clots and abnormal bleeding. The trial did not show any sign of such problems, although experts say that safety issues can only be detected in the real world, once a drug or vaccine has been given to millions of people.

The interval between the first and second dose can appear as a blessing for India as it will help the Centre and state governments to vaccinate more people belonging to the priority group. NTAGI’s Dr Arora said, "Now there is flexibility… you can get the vaccine anytime between 28 to 56 days.”