(Representational Image)
(Representational Image)

Imagine a vaccine that is 90% effective at preventing COVID-19 infection, is cheap and can be stored in a common fridge.

AZD1222, Oxford University's coronavirus vaccine has promised to be all of these and more, unlike its competitors. Yet the varsity's research partner for the shot, AstraZeneca, announced on Monday that the vaccine has shown two remarkably different efficacy rates resulting from two different dosing regimens.

Differing efficacies for differing dosing regimens

One dosing regimen showed vaccine efficacy of 90% when AZD1222 was given as a half dose, followed by a full dose at least one month apart, and another dosing regimen showed 62% efficacy when given as two full doses at least one month apart. The combined analysis from both dosing regimens resulted in an average efficacy of 70 per cent.

The pooled analysis included data from the Phase II/III trial in the UK and Phase III trial in Brazil. Over 23,000 participants are being assessed following two doses of either a half-dose/full-dose regimen or a regimen of two full doses of AZD1222 or a comparator, meningococcal conjugate vaccine called MenACWY or saline.

The global trials are evaluating participants aged 18 years or over from diverse racial and geographic groups who are healthy or have stable underlying medical conditions.

AstraZeneca is intent on giving the world a weapon to fight the global pandemic with a vaccine that is potentially cheaper to make, easier to distribute, and faster to scale-up than rivals — but what are we to make of the different efficacy rates?

Global race to best COVID-19 vaccine efficacy

This comes amid a global race among COVID-19 vaccine makers to best each other in terms of efficacy rate.

Russia's Sputnik V vaccine has been found to be more than 95% effective against Covid-19 in the second interim analysis of Phase 3 clinical trial data, its developers said on Tuesday.

The analysis was based on data from volunteers obtained 42 days after the first dose -- which corresponds with 21 days after the second dose, Gamaleya National Research Centre for Epidemiology and Microbiology of the Russian Health Ministry and Russian Direct Investment Fund (RDIF), said.

And based on data obtained 28 days after administering the first dose - seven days after the second dose, the efficacy of the Sputnik V vaccine was found to be 91.4%.

Amid a global race to come out with an effective vaccine to check the pandemic that had ravaged the world, Pfizer Inc and its German partner BioNTech too had announced that their vaccine appears 95% effective at preventing mild to severe COVID-19 disease in a large, ongoing study.

US-based biotechnology giant Moderna also announced that its COVID-19 vaccine has shown to be 94.5% effective in preventing the deadly disease.

What explains the different COVID-19 efficacy rates?

The varying efficacy rate for AZD1222 is "scientifically intriguing", experts have said. Although all results are statistically significant, and more data will continue to accumulate and additional analysis will be conducted, researchers are trying to comprehend the efficacy reading and establish the duration of protection.

One explanation for the results lies in the fact that the Russian vaccine uses two different vectors based on the human adenovirus, rather than using one and the same vector for two doses.

On the other hand, the AstraZeneca vaccine uses a more traditional approach to vaccine manufacturing — using a cold virus (one that infects chimpanzees instead of humans, in this case) to evoke an immune response. It could be the case that the cold virus is resulting in a strong response for that particular strain, potentially dampening the response for the coronavirus itself. Whereas, if the initial dosage is halved, experts think the molecular Trojan (technically a "vector") will potentially dampen the immune response for the cold virus and evoke a stronger response for the coronavirus instead.

Both Moderna and Pfizer's vaccines, however, use a more advanced, but less looked into, field of vaccine research — the mRNA genetic sequence on the SARS-CoV-2 coronavirus — to bring about their results.

What is the ideal vaccine efficacy rate?

As mentioned above, a COVID-19 vaccine with a higher efficacy rate does not necessarily mean that it is automatically the safest vaccine around, nor does it determine the ease of distribution or storage.

A vaccine with a lower efficacy rate but with a higher wiggle room for storage temperatures might end up being more preferable for nations to be disseminated to their populations, instead of relying on another potential candidate that requires, say, ultra-cold supply chains and logistics beyond what's possible for the pre-existing infrastructure in many underdeveloped and developing nations.

For viral vaccines in general, infectious diseases experts aim for an efficacy rate of at least 70%. It is usually presumed that a 60% effective vaccine is usable, but might still see some localised outbreak. Meanwhile, vaccine candidates with less than 60% efficacy rate will potentially fail to create enough herd immunity to stop the virus.

Covaxin, India's indigenously developed COVID-19 vaccine is expected to be at least 60% efficacious, and certainly more than 50% so, the Hyderabad-based Bharat Biotech said in a statement earlier this week.

Where is India in all this?

Five vaccines are under different phases of clinical trials in India. The Serum Institute of India is conducting the phase-three trial of the Oxford-AstraZeneca COVID-19 vaccine, while Bharat Biotech and ICMR has already started the phase-three trail of the indigenously developed COVAXIN jab.

An indigenously developed vaccine by Zydus Cadila has completed phase-two clinical trial in the country. Dr Reddy's Laboratories will soon start combined phase two and three trials of the Russian COVID-19 vaccine Sputnik V in India.

Prime Minister Narendra Modi on Tuesday said any vaccine against coronavirus that is administered to citizens will meet all necessary scientific criteria and stressed that governments at all levels will have to work together to ensure that the vaccination drive is smooth, systematic and sustained.

The prime minister also urged states to send their detailed plans on vaccine administration, underlining that a final vaccine distribution strategy will be chalked out in collective coordination with states.

(With inputs from agencies)

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