Decoded: How Covid-19 vaccine efficacy helps to get an idea of protection

With 81 per cent efficacy in the interim findings, Bharat Biotech’s Covaxin has got a shot in the arm to dispel doubts about a vaccine that is among the two being administered as part of the nationwide inoculation drive.

What is the meaning of vaccine efficacy?

Vaccinology talks about minimum essential function or the MEF of a vaccine, which includes safety and efficacy. Efficacy of a vaccine means its ability to stimulate immunity in a person who has received the shot. It is meant to significantly reduce the probability of disease — mild, moderate or severe — in a person exposed to the infection.

What is significant reduction? In case of the Covid-19 vaccine, the World Health Organization has kept it at 50 per cent. Another phenomenon that has not been measured is whether vaccination shifted the severity of the disease downwards. In the UK for instance, it has been found that vaccination reduced deaths.

How do you measure vaccine efficacy?

It is a mathematical calculation that compares the number of persons who got the disease in the two groups — the ones who received the vaccine and those who got the placebo. In case of Bharat Biotech’s Covaxin, 36 of the 43 cases were recorded in participants who received a placebo, compared with seven cases in people who were given the Bharat Biotech vaccine, pointing to an efficacy rate of 80.6 per cent. A majority of the cases were in the placebo group, which points towards the efficacy of the vaccine.

Bharat Biotech’s study covered 2,433 people over the age of 60 and 4,500 with co-morbidities. The interim analysis included a preliminary review of the safety database, which showed that severe, serious, and medically attended adverse events occurred at low levels and were balanced between vaccine and placebo groups.

What is its significance for India’s vaccination drive?

Experts say that the effectiveness of the vaccination programme is likely to be improved by good efficacy numbers. “When you roll out the vaccine the reduction in community transmission would be higher than what was shown in the efficacy data, as vaccinated people may transmit the disease much less,” said T Jacob John, retired professor and head, departments of clinical virology and microbiology, Christian Medical College.

He said that this is true for diseases where the transmission is human to human, unlike diseases such as tetanus. The data gives an idea of the protection that can be expected from the vaccine and also addresses vaccine hesitancy to a great extent.

How does Covaxin compare with other Covid-19 vaccines for which efficacy data is available?

Compared to Covishield, which showed around 54 per cent efficacy if the two doses were administered four weeks apart, Covaxin’s efficacy data is superior with much higher efficacy at the same interval in doses.

However, the efficacy of Covishield increases to almost 79 per cent if the second dose is given after a gap of 12 weeks. The regulatory approval for this has not been given yet.

Pfizer and BioNTech’s BNT162b2 vaccine has shown an efficacy of around 95 per cent. The vaccine is yet to be available in India. So is the vaccine made by Moderna and the US National Institute of Allergy and Infectious Diseases, with an efficacy of around 94 per cent. The single dose vaccine of Johnson & Johnson has shown an overall efficacy of 66 per cent in its interim findings, though the number differs in the US, Latin America and South Africa.

Russia’s Sputnik V has also claimed a high efficacy number of 91 per cent. It has been tested in phase 2/3 clinical trials in India by Dr Reddy’s Laboratories on around 1,500 volunteers.
 

With Covaxin, it is for the first time that an Indian indigenous Covid vaccine has proven its efficacy based on a sample size of over 25,000 volunteers. Covishield vaccine’s efficacy data relies largely on UK and Brazil studies.

What to expect beyond the interim efficacy data?

Bharat Biotech has said that the clinical trial will continue through to the final analysis of 130 confirmed cases in order to gather further data and to evaluate the efficacy of Covaxin in additional secondary study endpoints. Some experts feel that the delay in giving permission to start the phase 3 trial of Covaxin led to much lower confirmed cases. “Phase 3 started at a time when the cases in the country were on a decline. The regulator wanted the company to fully analyse phase 2 before moving on to the third one. The manufacturer should have got the permission way ahead of time,” John added.