With over 15 million confirmed cases of the new coronavirus, SARS-CoV-2, and more than 624,000 COVID-19 deaths globally, scientists around the world are competing against time to fast-track the development of new treatments to combat the disease.
Globally, scientists are developing about 250 candidate vaccines against SARS-CoV-2 infection. Of these, to date, at least 17 are under evaluation in clinical trials.
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The authors of the recent study aimed to evaluate the safety and efficacy of a vaccine candidate called adenovirus type-5-vectored COVID-19 vaccine (Ad5-vectored COVID-19 vaccine).
They also wanted to determine the most appropriate dose for a phase III trial in the coming months. They published their results in The Lancet.
The Beijing Institute of Biotechnology in Beijing, China, and CanSino Biologics developed the Ad5-vectored COVID-19 vaccine. It uses a weakened human common cold virus (adenovirus) that produces an immune system response, including antibodies, to fight off the coronavirus.
Danny Altmann, Professor of Immunology at Imperial College London in the United Kingdom, who was not involved in the study, told the Science Media Centre in London:
“The Beijing approach is based on the backbone of a conventional human, common cold virus to which some people have preexisting antibodies, and they, therefore, make a lower response in some people to the vaccine because people have preexisting antibodies to their vector, so may clear it before it has a chance to work properly.”
For the randomized controlled study, which took place in Wuhan, China, the researchers recruited and screened participants for eligibility in April 2020. In total, 508 participants (50% male) were eligible. The average age of the participants was 39.7, with 61% of them aged 18–44, 26% aged 45–54, and 13% aged 55 or older.
Of the 508 participants, 253 received a high dose of the vaccine, 129 received a low dose, and 126 received a placebo. The researchers observed the participants for 30 minutes after the injection to check for immediate adverse reactions and then followed them for any injection site or systemic adverse reactions within 14 and 28 days of the injection.
Additionally, the scientists took blood samples immediately before the treatment and at days 14 and 28 afterward to measure antibody responses.
The results revealed that 95% and 91% of those in the high dose and low dose groups, respectively, demonstrated either T cell or antibody immune responses 28 days after vaccination.
Specifically, at day 28, the team discovered T cell responses in 90% and 88% of the participants receiving the vaccine at a high and low dose, respectively.
Furthermore, both doses of the vaccine induced significant neutralizing antibody responses (in 59% and 47% of participants in the high and low dose groups, respectively) and binding antibody responses (in 96% and 97% of participants in the high and low dose groups, respectively) by day 28.
Participants in the placebo group, however, showed no antibody increases from baseline.
Although more people in the vaccine group experienced adverse reactions compared with those in the placebo group, most of the adverse reactions were mild or moderate. Adverse reactions included fatigue and pain at the injection site, and the most common severe reaction was fever.
Among the participants, 52% had high preexisting immunity to the Ad5 vector, and 48% had low preexisting immunity. Those with low preexisting anti-Ad5 immunity had neutralizing antibody levels that were approximately two times higher than those of the participants with high preexisting anti-Ad5 immunity.
Moreover, participants aged 55 or older had a significantly lower immune response than younger participants but higher tolerability to the Ad5-vectored COVID-19 vaccine.
“Since elderly individuals face a high risk of serious illness and even death associated with COVID-19 infection, they are an important target population for a COVID-19 vaccine. It is possible that an additional dose may be needed in order to induce a stronger immune response in the elderly population, but further research is underway to evaluate this,” states senior author Prof. Wei Chen, of the Beijing Institute of Biotechnology.
The current trial does have certain limitations. For instance, the team did not expose any of the participants to SARS-CoV-2 after vaccination.
Consequently, it is not possible for the current study to determine whether the vaccine candidate successfully protects against SARS-CoV-2 infection or whether there are any associated risks when the antibody (that the vaccination induces) encounters the new coronavirus.
Also, the trial only followed participants for 28 days. Therefore, the study has no data available about the resilience of the vaccine-induced immunity.
The authors conclude, “the results of this trial have extended our knowledge of the immunogenicity and safety of the Ad5-vectored COVID-19 vaccines.” They also add, “We are planning an international multicenter, randomized, double-blind, controlled phase III effectiveness trial to further evaluate the efficacy of the vaccine.”
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