The race for a Covid-19 vaccine has taken on critical importance as the disease continues to charge through the global population, with more than a million new infections every week and the death toll climbing.
The vaccine has been seen as a way out of the crisis and new results from some of the teams leading development are showing early promising signs. But the real test lies ahead in the final-phase trials, experts say.
The new data out on Monday includes results from a vaccine candidate produced by a team at Oxford University, working in partnership with British firm AstraZeneca.
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The candidate was found to be safe and induced strong immune responses in combined phase-one and phase-two trials, published in The Lancet scientific journal.
A candidate from Chinese biopharmaceutical company CanSino Biologics, produced in collaboration with the Chinese military, was also found to be safe and able to produce an immune response in most candidates, according to results from phase-two trials, also published in The Lancet on Monday.
A second set of preliminary results for a vaccine candidate produced by United States company Pfizer and German partner BioNTech were released as non-peer reviewed data on Monday. This candidate too produced a strong antibody response.
While some side-effects were noted in all three candidates, none were classified as severe.
Mike Ryan, executive director of the World Health Organisation’s Health Emergencies Programme, welcomed the data release as “good news” in a press briefing on Monday, but warned that “there is a long way to go” as candidates moved into “real-world, larger phase trials”.
Early-stage trials look to evaluate issues such as the safety, dosing and side-effects of a vaccine candidate, while checking for efficacy in terms of producing a potentially virus-neutralising immune response.
Phase-three trials, the last phase before possible regulatory approval, continue to monitor for safety while evaluating how effective a vaccine is at protecting people from getting sick in their daily lives. These trials often aim to include people from diverse demographics and can have sample sizes in the tens of thousands.
Immunologist Kylie Quinn, a vice-chancellor's research fellow at RMIT University in Australia, said the latest round of results, indicating that candidates were producing the immune response researchers were looking for, marked a “crucial hurdle” on the path towards a vaccine.
“When they can get over that hurdle, then it makes it worthwhile to do a phase-three trial to really test if [the candidates] can work in a general population,” she said.
But tempering expectations was also important, Quinn said, as approved vaccines might not be able to completely protect people from the disease, but could still “provide benefits in reducing disease severity and the transmission of the virus from person to person”.
Several phase-three trials are already ongoing or expected to begin this month, and predictions about when potentially approved vaccines could be ready for market range from the end of this year to mid-2021.
Two vaccine candidates produced by Chinese teams are in final-phase trials, with one by Chinese firm, Sinopharm, in trials in Abu Dhabi, and another by Sinovac Biotech’s being the subject of trials in Brazil. CanSino earlier this month said that it was in talks for its overseas phase-three trials.
Low numbers of infection in China have made clinical trials in the country difficult.
The vaccine candidate from Oxford University and AstraZeneca is already in phase-three trials in Brazil and South Africa, with enrolment open in the US for a large-scale trial there.
US company Moderna, working with the US National Institute of Allergy and Infectious Diseases, plans to start final-phase trials at the end of the month. The team last week published its phase-one results showing its product was safe and produced immune responses.
The Pfizer and BioNTech candidate may also enter late-stage testing this month, pending approval.
Continuing trials could be an opportunity for researchers to understand how the vaccine works among a more diverse set of people and how to optimise dosing.
Referring to the Oxford vaccine results, virologist Ian Mackay, an associate professor at Australia’s University of Queensland, said for the next phase trials “we'd hope to see some less healthy, older and much more diverse vaccinees [vaccinated subjects] – more reflective of the group most at risk of severe disease and death”.
“There are many places worldwide with lots of cases so that effect should be relatively quick to see in future trials,” he said.
The researchers doing trials with the CanSino vaccine removed the age cap for their phase-two trial, which included 508 participants. They found that older people had a “significantly lower immune response”, to their vaccine, which uses a modified cold-causing adenovirus as its platform.
They suggested high pre-existing immunity to that cold virus may be the cause, but that an additional booster dose “might be a potential solution to provide enhancement of immune responses” for such populations.
In the Oxford candidate trial, participants who received the vaccine had neutralising antibodies, with the strongest responses detected after a booster dose was given to a small subset of the 1077 participants.
Immunologist Ashley St John, an assistant professor involved in vaccine research at Duke-NUS Medical School in Singapore said “some approved vaccines even require six boosters to be fully effective, so it's encouraging to see a strong response after two”.
Findings that a high proportion of volunteers developed antibodies that could neutralise the virus in cell culture were “promising” she said, but “not the same as testing efficacy in humans, which the follow-up studies will do”.
“But it is encouraging as a first step,” she said.
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