While the second wave of the pandemic extends its claws, India continues waging a war against the virus by inoculating its population. Currently, India has granted emergency use authorization to three COVID-19 vaccines — Covishield, Covaxin, and Sputnik V (Russian vaccine). While Sputnik V is still awaited to hit the healthcare settings, Covaxin and Covishield have helped in the successful immunization of more than 17 crore people, of which, more than 3 crore are fully vaccinated.
Covishield and Covaxin both need two doses to complete the vaccination schedule. On 13th May 2021, the Ministry of Health and Family Welfare announced that the gap between the two doses of Covishield vaccine has been revised. While earlier the doses were administered 6-8 weeks apart, now the second dose of Covishield will be given 12-16 weeks after the first dose .
Please note that there was no change suggested for Covaxin and the gap between the two doses continues to be 4 weeks or 28 days.
Who Suggested The Change Between Two Doses of Covishield And Why?
The Union Health Ministry in its COVID briefing said that this decision was made based on the recommendation made by the National Technical Advisory Group on Immunisation (NTAGI) .
In January, the recommended gap between two doses of the Covishield vaccine was 4-6 weeks. This was based on the data available during the initial clinical trials. However, as more data became available through secondary analysis, this gap was increased to 6-8 weeks in March. Now, as per recent emerging real-life evidence, particularly from the UK, it is seen that the efficacy of the vaccine further peaked if the duration between the first dose and the second dose of Covishield vaccine was increased to 12-16 weeks.
National Expert Group on Vaccine Administration for Covid-19 (NEGVAC) finally accepted the recommendation made by NTAGI after thorough discussions with experts from the World Health Organization (WHO).
This decision was also supported by Adar Poonawala, CEO of Serum Institute of India, the manufacturer of Covishield. He was reported to have said that this increased gap between the two doses will enhance the efficacy and also benefit the immunogenicity.
How And Why Revising The Gap To 12-16 Weeks For Covishield Makes It More Effective?
The first dose of a vaccine (called the prime dose) evokes your immune system and leads to the activation and proliferation of the body’s defense cells. The number of these cells increases after the prime dose, peaks and then declines due to cell death. As observed in a computational model of the immune system conducted for vaccines in general, the defense cells are more abundant on days 15 and 45 than on day 90 after getting the first shot. The second dose of a vaccine called the booster dose is optimal at a time point when the number of defense cells produced by the primary dose has declined. It is important here to emphasize that the second dose exposes the remaining defense cells to the viral antigen and leads to an increase in their lifespan. It has been observed that a time interval of several weeks between the first and second vaccine doses can be necessary to obtain optimal immune system responses .
Different vaccines have different needs with respect to the minimum interval between subsequent doses. As more and more data becomes available around COVID vaccines and new studies perform deeper analyses of the vaccine dynamics, the gap between two doses can be optimized.
As per a Lancet study , the efficacy of the Covishield vaccine increased when the shots were administered at a gap of 12 or more weeks. The numbers suggest that the efficacy was around 81.3% when the second dose of Covishield was given at an interval of 12 or more weeks. Whereas, the efficacy was around 55·1% when given at an interval of less than 6 weeks.
What If You’ve Taken The First Dose Of Covishield And The Second Dose Is Pending?
– As per the latest recommendation, you should take your second dose of Covishield after 12-16 weeks of the first dose.
– In case you’ve already booked an appointment for your second dose, then it shall remain valid . However, it is advised that you reschedule your vaccine appointment for a day 84 days after the first dose.
– From now on, online or on-site registration for the second dose of Covishield will not be possible if the duration after the first dose is less than 84 days .
– If you have pre-booked your slot and still wish to take the dose before 84 days of the first dose, Cowin will allow .
Should People Who Have Taken It At Intervals Of 4-6 And 6-8 Weeks Be Worried?
Not at all. Data shows that within 4 weeks of taking the first dose of Covishield, the chances of hospitalization are reduced by 94% . The second dose of Covishield is the booster shot that helps bolster the immunity kickstarted by the first dose.
What About The Confusion Between Different Dose Regimens For Covishield?
The overall efficacy of the Covishield after two weeks of completing both prime and booster doses was 70%. However, this number was based on averaging the results from two groups. While the first group that was given two full doses demonstrated to be 62% effective, the second group that received a half dose followed by a full one came to be 90% effective.
A Possible Explanation: The Vaccine Design Could Affect Efficacy
Although it is not yet known why the low-dose regimen of the Oxford vaccine showed better efficacy in trials, a possible logic points towards the viral vector.
On receiving a viral vector vaccine, the body generates an immune response against the coronavirus’s spike protein as well as the viral vector itself. As a result of this, when the booster dose is given, the body may destroy some of it. This is a well-known limitation of viral vector vaccines.
A possible reason why the lower first dose of the Oxford vaccine followed by a full one came to be 90% effective could be that when a lower first dose was given the anti-vector immune response developed by it was not strong enough to impact the booster dose. As a result, both the doses initiated a robust immunity and had a greater overall efficacy.
Does Higher Efficacy Mean Leniency With Masks And Social Distancing?
Absolutely No! Vaccine efficacy is the percentage of reduction in disease incidence in a vaccinated group compared to an unvaccinated group under ideal and controlled conditions. Simply put, it measures how many vaccinated people got infected with the virus as compared to the unvaccinated group during the trial.
To calculate the vaccine efficacy, researchers compare the number of vaccinated people who got COVID-19 to the number of unvaccinated people who contracted it. This difference in risk can be calculated as a percentage which is called the efficacy. So, for example, if a vaccine’s efficacy is 0 then it means that both vaccinated and unvaccinated people share equal risk of contracting the disease. Whereas, an efficacy of 100% means the risk of getting the disease is nil in vaccinated people. Efficacy varies depending on outcomes considered, where the trial took place, etc.
Moreover, keep in mind that no vaccine is 100% effective against infection. They help reduce the risk of severe disease and hospitalization if infected. Trust the vaccines and get jabbed. But, make sure to follow COVID-appropriate behavior at all times even after getting vaccinated- wear masks, maintain physical distancing, and wash hands often.
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(The article is reviewed by Dr. Swati Mishra, Medical Editor)
1. Press briefing on the actions taken, preparedness and updates on COVID-19, Dated: 13.05.2021. (timestamp: 48:28) https://www.youtube.com/watch?v=ijLcZ8G84FI
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4. Ministry of Health and Family Welfare. Twitter. https://twitter.com/MoHFW_INDIA/status/1393975954322595844/photo/1
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