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The Curious Case of Vaccine Efficacy, and the Unanswered Questions Behind It

The Curious Case of Vaccine Efficacy, and the Unanswered Questions Behind It

A researcher holds a vial of the Sputnik V vaccine candidate at a facility in Budapest, November 19, 2020. Photo: Matyas Borsos/Hungarian Foreign Ministry.

Within a year of emergence of the COVID-19 pandemic, more than 70 vaccine candidates have entered clinical trials around the world, with a dozen in phase 3 trials at the moment.

Typically, phase 3 trials are designed to test vaccine efficacy and mark the final stage before seeking regulatory approval for public use.

In the past month, there has been a flurry of press releases announcing phase 3 efficacy results – from Pfizer, Moderna, AstraZeneca and the Gamaleya Institute in Russia. Quite amazingly, the reported efficacies for all these COVID-19 vaccine candidates have been much higher than anticipated, and all of them appear to be on the verge of obtaining emergency use authorisation.

Early on during the pandemic, even before phase 3 efficacy trials had been completed, the US, the UK and Europe scrambled to secure priority access to multiple promising vaccine candidates, leaving the remaining countries to fend for themselves. But SARS-CoV-2 is a virus that does not respect national borders and the global fight against it will be only as strong as the weakest link. This is where India can step in – in a critical role as the country with the largest capacity to manufacture COVID-19 vaccines, to strengthen the weaker links.

India is the world’s third largest producer of pharmaceuticals by volume. Developing countries rely chiefly on India for their medicine supplies. Nearly 50% of essential medicines supplied by UNICEF, and more than 75% of all medicines supplied by the International Dispensary Association to developing countries, are manufactured in India.

India’s contribution to the global war on the pandemic works at two levels: One, by leveraging its capacity to manufacture foreign vaccines at a large scale, and two, by developing and manufacturing indigenous vaccines. Historically, India’s strength has been in the former, and is taking baby steps in the latter by seeking to enable research and innovation.

As a result of India’s efforts at both levels, about half-a-dozen different COVID-19 vaccine candidates are in advanced stages of clinical development today. Covaxin, Covishield and Sputnik V are the current frontrunners with others, namely ZyCoV-D, Covovax and BECoV-2, following on their heels. A host of other candidates are in the early stages of development.


This is an indigenous vaccine candidate based on inactivated SARS-CoV-2 viruses isolated from an infected Indian patient. The Indian Council of Medical Research (ICMR) provided the virus-isolate to the Hyderabad-based vaccine manufacturer Bharat Biotech (BB) in May. The company cultured the virus at its facility, purified and chemically inactivated it to obtain the candidate known as Covaxin. This will be administered in two doses.

BB is currently running two combined phase 1/2 trials of Covaxin to test its safety and capacity to stimulate immune responses against the virus. The first one was initiated in July and the second one in September. Collectively, these two trials will test Covaxin in over 1,200 participants.

So far, no interim results have been posted in the public domain. The company also launched a phase 3 trial of Covaxin in November, which plans to enroll 26,000 volunteers across 25 sites in India. BB currently has a limited capacity to produce Covaxin but expects to scale it up to a billion doses soon and launch it in mid-2021. The company predicted that Covaxin’s efficacy would be about 60%. The basis for this estimate is unknown.


The Pune-based vaccine company Serum Institute of India (SII) is developing the Oxford University/AstraZeneca COVID-19 vaccine candidate, AZD1222, under the brand name ‘Covishield’. AstraZeneca struck a licensing agreement with SII in June for the supply of one billion doses of its COVID-19 vaccine for low and middle-income countries, with a commitment to provide 400 million doses before the end of 2020.

To create AZD1222, Oxford University scientists took a respiratory virus, of the type adenovirus, which normally infects chimpanzees, weakened it to make sure it doesn’t multiply and redesigned it to encode the spike (S) protein of SARS-CoV-2. AZD122, which is given in two doses, is currently in phase 2/3 trials in the UK and phase 3 trials in the US and several other countries.

On the basis of results of AZD1222 phase 1/2 trials abroad, the Drugs Controller General of India (DCGI) permitted SII to carry out phase 2/3 trials of Covishield in India. SII launched the phase 2/3 trial in the last week of August, in collaboration with ICMR. This trial, involving 1,600 participants, is designed to determine Covishield’s safety and capacity to induce an immune response to SARS-CoV-2, but not its efficacy.

SII expects to scale up production of Covishield to 100 million doses by the year’s end. The company also predicted the availability of Covishield to the general Indian public in April 2021, at Rs 500-600 a dose.

AstraZeneca’s announcement of AZD1222 efficacy as either 90% or 62%, based on unintentionally varying its dosing regimen, has generated confusion and doubt in the international scientific community. The Indian government has decided that the initially intended dosing regimen with 62% efficacy is acceptable, for unexplained reasons.

Soon after, SII announced its intention of seeking emergency use authorisation for Covishield by mid-December.

Also read: Why AstraZeneca Is Facing Tricky Questions About Its COVID-19 Vaccine

Sputnik V

Sputnik V is also based on the adenovirus platform, but uses adenoviruses that normally infect humans, not chimpanzees. It actually uses two types of human adenoviruses – type 5 and type 26 – for its two doses. Sputnik V was developed by the Gamaleya Institute in Russia, and approved in the country in August, well before initiating phase 3 efficacy trials.

In mid-September, Dr Reddy’s Labs, a Hyderabad-based drug company, entered into an agreement with the Russian Direct Investment Fund to conduct clinical trials of Sputnik V, which is currently in phase 3 trials. Based on an interim analysis two months into the phase 3 trial, Russia announced that Sputnik V’s efficacy is more than 90%.

In October, the DCGI permitted phase 2/3 trials of the Sputnik V candidate to be conducted in India. This trial, which plans to enrol 100 participants in the phase 2 stage and 1,500 participants in phase 3, has just started.


The Ahmedabad based drug company Zydus-Cadila is also developing an indigenous COVID-19 vaccine candidate. Unlike the others, this one is based on a circular molecule of DNA (known as a plasmid) containing the genetic information for making the SARS-CoV-2 virus’s S protein. This candidate, called ZyCoV-D, completed phase 1 testing in the first week of August and is currently in phase 2. The company expects to have phase 2 trial results soon and plans on launching phase 3 trials in December.


SII has tied up with the US-based vaccine company Novavax to develop a protein based COVID-19 vaccine. The Novavax candidate, known as NVX-CoV2372, is based on the SARS-CoV-2 S protein, displayed on microscopic particulate scaffolds.

NVX-CoV2372 is currently in phase 3 trials in the UK, with interim results expected in April 2021. This protein-vaccine candidate will be manufactured and formulated by SII, under the brand name Covovax. Phase 3 trials of Covovax in India will be supported by ICMR.


On August 13, Biological E, a Hyderabad-based vaccine and drug maker, entered into a licensing agreement with the Baylor College of Medicine (BCM) in the US, to develop another protein based COVID-19 vaccine candidate.

The BCM vaccine licensed to Biological E contains a part of the SARS-CoV-2 spike protein, known as RBD (receptor-binding domain), but no genetic material. Biological E initiated a phase 1/2 trial of this vaccine candidate, which it has named as BECoV-2, in mid-November. This trial will enrol 360 participants to test BECoV-2 for safety and its capacity to induce antibodies to SARS-CoV-2, and is expected to be completed by end of 2021.

Other candidates

Aside from these COVID-19 vaccine candidates, an assortment of additional candidates are in relatively earlier stages of development. BB has inked an agreement with the Washington University Medical School to manufacture and conduct clinical trials of a single-dose intranasal vaccine, also based on a chimpanzee adenovirus.

Biological E has a tie-up with Johnson & Johnson to manufacture the latter’s human adenovirus-26 based candidate, Ad26.COV2.S.

SII is working to develop a weakened but live SARS-CoV-2 vaccine candidate, in collaboration with the US company Codagenix. The company is also collaborating with Spybiotech, an Oxford University spin-off company, to develop yet another protein vaccine. This vaccine contains RBD part of SARS-CoV-2 displayed on non-infectious virus-like particles.

A heat-tolerant version of RBD is under development by Mynvax, a Bengaluru-based startup.

Pune-based Gennova is working in collaboration with a US based company to develop an mRNA vaccine candidate with improved heat stability.

Unaddressed concerns

It is understandable that developing vaccines during a pandemic is fraught with challenges and glitches. Many COVID-19 vaccine candidates are being developed using novel but unproven vaccine technologies. Vaccine developers and health department authorities have continued to insist that safety and efficacy will not be compromised. But it is not clear how safe and efficacious the vaccines that are now on the verge of emergency approval will actually turn out to be.

All the leading COVID-19 vaccine candidates have revealed safety signals that can’t be ignored. The first was an instance of transient but serious adverse event (SAE) in four of the 45 participants in Moderna’s phase 1 trial.

AstraZeneca’s AZD1222 trials around the world were temporarily halted when a second case of SAE was reported in the UK trials. This SAE involved severe spinal inflammation, a condition known as transverse myelitis.

More recently, a participant in the Indian Covishield trial allegedly suffered an SAE linked to inflammation of the brain.

Also read: COVID-19 Vaccine: A Clinical Trial Participant Falls Sick. What Happens Next?

These events warrant a thorough investigation of this platform technology. COVID-19 is widely believed to have been the outcome of SARS-CoV-2 breaching the animal-human species barrier. With a live, albeit weakened, chimpanzee virus as a vaccine vehicle, we are now consciously breaching the same barrier. We have no idea of the long-term repercussions.

In another instance, a case of unexplained illness was reported in Johnson and Johnson’s trial of their Ad26 based COVID-19 vaccine candidate. A final instance of SAE was viral pneumonitis (lung inflammation) in the Indian Covaxin trial.

One possible explanation may be that this participant contracted a SARS-CoV-2 infection around the time of receiving the vaccine candidate – at a time when Covaxin hadn’t yet had time to act. Alternatively, this SAE may be linked to incomplete inactivation of the virus in the Covaxin preparation. We do not know the precise answer.

In all cases, the SAEs have been shrouded in secrecy. In some, trials continued despite the occurrence of SAEs while in others they were paused, and later resumed, because the events were determined to be unrelated to the vaccine candidate being tested. Again, we don’t know the basis for such determination.

By definition a COVID-19 vaccine must prevent people from falling severely sick and dying of COVID-19. Therefore it needs to be effective in saving lives. The recently announced efficacy results were based on efficacy trials designed to find if the vaccine candidate prevented mild, but not severe, COVID-19 symptoms. This kind of trial design was presumably dictated by the rapid pace. In this design, essentially any vaccinated individual who did not display cough and sore throat would be counted into the efficacy estimate. The fact that a huge majority of SARS-CoV-2 infections are asymptomatic actually contributes to overestimation of vaccine efficacy.

Thus, these results can’t be the basis on which we can predict if a vaccine candidate is capable of preventing people from falling severely sick and dying.

Ultimately it is not possible to choose one of the current crop of leading candidates over the other based on the available efficacy data. It is likely that one or more of these candidates may display acceptable levels of effectiveness, preventing transmission and death, in the coming months.

What could be a likely choice?

As the COVID-19 vaccines are to be given to tens of millions of people, it would be a safer bet, especially for at-risk groups, to go with a vaccine based on proven technologies. Inactivated vaccines and protein-based vaccines are safer because they are non-infectious. But the tradeoff is that they don’t seem to be as effective as live viral vaccines in stimulating immune responses.

Between the inactivated and protein based vaccines, the latter are the safer option, as incomplete inactivation of a live virus can cause the infection the inactivated vaccine seeks to prevent. In practice, a weaker immune response produced by inactivated and protein vaccines are addressed by the inclusion of additives, called adjuvants, which augment vaccine potency, and by administering booster shots.

The S-protein based Covovax (SII) and the the S-protein RBD based vaccine candidates (SII and BE) may offer an acceptable compromise of the multiplicity of factors, including safety, efficacy, affordability and thermal tolerance, that go into deciding the most suitable vaccine option. But these candidates are currently lagging behind those based on novel vaccine technology.

S. Swaminathan is a retired scientist based in New Delhi. The views expressed here are the author’s own.

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