- Scientific evidence shows that administering the second dose of COVISHIELD between 6-8 weeks, but not later than the 8-week period, improves protection.
- Previous studies have found that a longer interval of up to 12 weeks is more effective, which is backed up by immunogenicity data.
In its ongoing vaccination program against Covid-19, the government has decided to extend the time between the first and second doses of Covishield to up to two months. Covishield is the Serum Institute of India’s version of AstraZeneca’s AZD1222, a vaccine developed in collaboration with the University of Oxford by AstraZeneca.
According to data from global AZD122 trials, extending the time between doses to 12 weeks significantly increased its effectiveness. Interim results from trials in the United States, Peru, and Chile, found that the vaccine had a 79 percent effectiveness even when the second dose was given four weeks after the first.
On the recommendation of two expert groups for Covid-19, the National Expert Group on Vaccine Administration (NEGVAC) and the National Technical Advisory Group on Immunization (NTAGI)– the Ministry of Health and Family Welfare made the decision. After reviewing available scientific evidence from vaccine clinical trials, the groups found that the vaccine’s protection against Covid-19 is “enhanced” if the second dose is given between 6 to 8 weeks.
According to results from AZD1222 trials in other nations, the vaccine’s effectiveness increased when the second dose was given more than six weeks after the first. In this case, efficacy refers to the vaccine’s ability to reduce symptomatic Covid-19 cases in those who have been inoculated versus those who have not.
According to a study analysing February Covid-19 cases in phase 3 clinical trial participants across the UK, Brazil, and South Africa, AZD1222 effectiveness was around 54.9 percent when the second dose was given less than two months after the first dose. When the second dose was given 6-8 weeks after the first dose, the effectiveness improved to 59.9%, 63.7 percent when the second dose was given 9-11 weeks, and 82.4 percent when the dosing interval was extended to 12 weeks or more. This study has not yet been peer-reviewed, despite the fact that it was submitted to The Lancet in February.
Interim results from phase 3 clinical trials conducted on 32,000 participants across the US, Chile, and Peru show that the vaccine had a 79 percent effectiveness against symptomatic Covid-19 when the interval between doses was four weeks, according to Oxford University and AstraZeneca. More importantly, in cases of serious or critical symptomatic Covid-19, the efficacy was 100 percent.
The effectiveness found in these trials is significantly greater than that found in trials performed in the United Kingdom and Brazil.
According to a letter sent to all states and union territories by Health Secretary Rajesh Bhushan, expert groups analyzing the evidence concluded that the vaccine would not be able to provide improved protection if the dose interval was extended beyond two months.
According to Dr. N K Arora of National Immunization Technical Advisory Group (NTAGI), one of the organisations that studied the issue, there was “no good scientific evidence” to support a recommendation to extend the interval beyond two months, particularly since the vaccine is not in short supply in India. Dr. Arora further added, “We’ve examined every piece of evidence we can get our hands on and remain unconvinced.”
“This proposal (to extend the interval beyond two months) is intended for countries and societies where vaccine supplies are limited.” India occupies a singular position in the world. He continued, “We have enough vaccines.”
There is also a risk of “breakthrough infections” without any “significant benefit” in terms of efficacy that the groups identified with extending the dosing interval, particularly at a time when cases are on the rise in the country.
“Increasing the dosage interval (too much) increases the risk of a terrible infection developing between the two doses. So, if I give the first dose and then wait 12 weeks, there’s a chance that some people will contract Covid infection in the interim. Dr. Arora stated, “We don’t want that.”
Even if extending the dosing interval results in a higher level of antibodies, Dr. Arora cautioned that this does not necessarily imply that the patient will be better protected. He said, “There’s is no link between increased antibodies and improved protection.”
By delaying the second dose, more doses may become available, allowing a greater number of people to receive their first dose of the vaccine sooner. The government, on the other hand, believes that allowing this increase in interval will make vaccination easier for the priority group population, which primarily consists of the elderly.
According to NTAGI’s Dr. N. K. Arora, “Now there is some flexibility… The vaccine can be given at any time between 28 and 56 days.