Covaxin’s protective efficacy in monkeys doesn’t mean it will protect humans as well, say virologists

Hyderabad-based Bharat Biotech, which released data from Covaxin’s animal trials, stated that the vaccine prevented infection and disease in primates subjected to high amounts of exposure to live virus Sars-CoV-2. But virologists have asserted out that its protective efficacy in monkeys does not mean it would be equally protective for humans.

In Covaxin’s animal trials, 20 rhesus macaques were divided into four groups of five each. One group was administered a placebo while three groups were immunised with three different vaccine candidates at 0 and 14 days. The SARS-CoV-2 virus was introduced to all macaques fourteen days after the second dose.

The protective response was observed with increasing SARS-CoV-2 specific IgG and neutralising antibody titers from 3rd-week post-immunisation. Bharat Biotech also observed that no evidence of pneumonia was observed in the vaccinated groups. There was also reduced replication of the virus in the nasal cavity, throat, and lung tissues of the monkeys. The placebo group of monkeys, however, showed signs of pneumonia and localisation of the viral antigen in certain membranes of the lungs.

Virologists have cautioned against premature euphoria. “Protective efficacy in experimental animals is encouraging, but it is not equivalent to protection in human beings. One could expect the efficacy of the vaccine in humans as well, but the proof of the pudding is in eating it. It has to be proven in humans. It’s good news, but does not absolve the company from Phase II and Phase III trials,” said Dr T Jacob John, virologist and former professor at the Christian Medical College, Vellore.

Dr Sunit Singh, Professor of Molecular Immunology and Virology at Banaras Hindu University had a similar view. “This response in animals only tells that there is a probability of generation of protective response in humans. Of course, it cannot be guaranteed but this is the only way to move ahead. Protective efficacy must also be associated with safety,” he said.

The reason for caution is because other vaccines have also shown positive results in monkeys. The UK-based University of Oxford, in association with AstraZeneca, reported positive findings of its COVID-19 vaccine in a small study involving six monkeys. The COVID-19 vaccine being produced by the US-based Johnson and Johnson also led to robust protection against SARS-CoV-2 in rhesus macaques, its study researcher Dan H Barouch had stated. The Johnson & Johnson study involved 52 monkeys.

The vaccine being tested by US-based Moderna and National Institutes of Health was given to monkeys in two shots over four weeks. In some of the vaccinated monkeys, researchers could not detect the virus in the nose or lungs. In others, the virus replicated slowly before disappearing. Moderna is currently conducting Phase-III of its clinical trials.

Earlier this week, AstraZeneca paused phase-II clinical trials of its vaccine after suspected adverse reaction in a participant in the United Kingdom. This forced Serum Institute of India to also put on hold trials of the vaccine in India.

Bharat Biotech’s vaccine observation on monkeys is a step in the right direction, but SARS Cov-2 infection was not prevented either in the Oxford vaccine or in the Bharat Biotech vaccine, pointed out John. However, in both the cases, the infection in the monkeys was short-lived.

Recently, there have been news reports of COVID-19 patients getting re-infected after a couple of months. The vaccines are unlikely to halt that either. “Re-infection is a very different thing (only a few reports so far). Since it is an RNA virus (common cold, influenza, SARS, dengue, measles, Hepatitis C, polio, Ebola) there are always chances of generation of variants of SARS-CoV2 during each cycle of replication of this virus,” explained Singh.

When a vaccine is developed, the dominant conserved epitope of the virus is targeted to develop the vaccine. “But if there are any changes in that selected dominant conserved epitope then the vaccine might not be effective. This challenge is associated not only with the SARS-CoV2 but most of the RNA viruses,” added Singh.

These vaccine results on animals have come when India has registered 97,570 cases and the total number of cases stood at 46,59,984. The death count has risen to 77,472. India has the second-highest Coronavirus caseload in the world after the United States.