Cuba’s State-run corporation, BioFarma, said on Friday that its indigenously produced Soberana 2 vaccine was 91.2% efficacious in phase-3 trials. This follows closely on an announcement that another of its vaccines, Abdala, had reported an efficacy of 92.8% in late stage trials.
The greater-than-90% efficacy puts them in a select league; however, unlike the Pfizer-BioNTech and Moderna vaccines that had reported an efficacy of over 90%, both the Soberana and Abdala are three-shot vaccines.
Both are subunit vaccines, meaning that a part of the virus forms the antigen and is hitched on to another construct. In Abdala the spike protein of the coronavirus is combined with a chemically manufactured adjuvant, whereas in Soberana 2, the spike protein is chemically linked to the tetanus toxoid, making it a conjugate vaccine. The design and manufacturing allows the vaccine to be stored in regular refrigeration settings of 2–8 degree Celsius.
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Conjugate vaccines
The most common conjugate vaccines are those used for Haemophilus influenza type b and the pneumococcal bacteria. However, a unique aspect of the Soberana vaccine is that it is by far the only one among coronavirus vaccine candidates that relies on the conjugate vaccine technology.
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Marlene Ramirez Gonzales, one of the scientists involved in the Cuban vaccine development project, in a March letter to the British Medical Journal explained the rationale:
“The [Carribean] island’s four vaccine candidates against COVID-19 are developed as subunit vaccines, one of the most economical approaches and the type for which Cuba has the greatest know-how and infrastructure. From protein S - the antigen or part of the SARS-CoV2 virus that all COVID vaccines target because it induces the strongest immune response in humans - Cuban [vaccine] candidates are based only on the part that is involved in contact with the cell’s receptor: the RBD (receptor-binding domain) which is also the one that induces the greatest number of neutralising antibodies...Cuba had already developed another vaccine with this principle. It is Cheimi-Hib, ‘the first of its kind to be approved in Latin America and the second in the world’, against haemophilus influenzae type b, coccobacilli responsible for diseases such as meningitis, pneumonia and epiglottitis.”
Experts say that while there are no inherent disadvantages to taking a conjugate-vaccine approach for coronavirus vaccines, they have generally been used against bacteria and not viruses.
The two parts of a conjugate vaccine are typically connected by chains of polysaccharides, according to epidemiologist and public health expert, Chandrakant Lahariya, and they generally induce a weaker immune response in young children. “Cuba has a long history of vaccine development and has developed certain platforms that work to their advantage. So it makes sense for them to adopt this route. There is nothing inherently disadvantageous, though, to this approach,” he said.
Effective response
For an effective vaccine response, not only antibodies but even killer T-cells, or those produced by the immune system and capable of destroying infected cells, must be produced. In a protein sub-unit vaccine, the spike protein may be able to elicit a strong antibody response but when combined with the tetanus toxoid, a very widely used childhood vaccine and therefore one which the immune system recognises, such a T-cell response could also be generated and conferring more effective protection, said Shahid Jameel, virologist and Director, Trivedi School of Biosciences, Ashoka University.
He added that while there could be “logistic” concerns with a three-dose vaccine, it wasn’t right to compare efficacies of various vaccines as those numbers had different contexts. “Every vaccine's efficacy results are in comparison to placebo, and not against another vaccine.... each trial was done differently.”
While the efficacy results of the Cuban vaccines haven’t been published in peer-reviewed journals, that the vaccines have been developed entirely by the public health system and amidst a U.S. trade embargo, are among the reasons why they have evoked interest in several other Caribbean countries as well as beyond.
Global response
Iran’s Pasteur Institute has said it will participate in phase-3 clinical trials for Soberana 02, with another 60,000 to be enrolled in Venezuela. Other countries including Mexico, Jamaica, Vietnam, Pakistan and India have expressed an interest in the Cuban vaccines, as has the African Union (on behalf of all 55 of the African nations). Cuba, which exports medical services, has said it will apply different rates for vaccines depending on the importer’s ability to pay.