KUALA LUMPUR: Shortening the dosing interval for AstraZeneca vaccines anywhere from four to eight weeks after the first dose is preferred in an environment involving highly infectious variants such as Delta.
Epidemiologist Professor Datuk Dr Awang Bulgiba Awang Mahmud of Universiti Malaya said the Pfizer-BioNTech and AstraZeneca vaccines had shown to work against the Delta variant after two doses, but a single dose did not offer optimal protection.
Therefore, he said, reducing the dosing interval to six weeks struck a balance between trying to get the maximum level of antibodies and making sure that there was optimal protection against the Delta variant.
"A reduction in dosing interval to six weeks is also the approach adopted in Australia, which is facing a Covid-19 wave fuelled largely by Delta.
"I do not think the coverage would be affected. We are at the point where a majority of adults have already received two doses, save for some AstraZeneca vaccine recipients, who have yet to receive their second dose," he told the New Sunday Times.
The dosing interval for the AstraZeneca vaccine was originally set at 12 weeks following the United Kingdom government's recommendations. It was reduced to nine weeks in July amid the surge in infections in Malaysia.
Health Minister Khairy Jamaluddin on Friday said the window between the two jabs would be shortened further from nine weeks to six, effective Oct 1, to make the vaccination process easier for those who had been inoculated with AstraZeneca.
Dr Awang Bulgiba, who is the Science, Technology and Innovation Ministry's Covid-19 Epidemiological Analysis and Strategies Task Force chairman, said a third dose would likely be required for people in immuno-compromised segments of the population.
"They include cancer patients recovering from treatment or transplant patients on immunosuppressive drugs. If the B- and T-cell immunity of the rest of the population remains strong, then the need for repeated boosters may be obviated."
He said there was no need for people to prove that there was a drop in their immune response after taking the second dose.
"The Independent Covid-19 Vaccination Advisory Committee headed by me has been calling for a Voluntary Covid-19 Vaccination Registry since December last year.
"In July last year, the government announced the setting up of the sero-surveillance programme, which we hope will provide the evidence required for policy decisions of this type.
"Data from this registry need to include the levels of neutralising antibodies, and B- and T-cell (immunity) levels over time."
This data, he said, would help determine which groups needed a booster dose.
He added that a cost-benefit analysis (CBA) on the administration of third doses was crucial to inform whether the third dose should be provided free by the government or be an optional thing that should be paid out-of-pocket.
"For people whose immune system does not respond that well or are unlikely to have responded well to the first two doses, I do not think they should pay for their third dose. They should be given priority over others in getting the third dose.
"I urge the government to consult our health economists, many of whom are in universities and are well-placed to perform this CBA."
Molecular virologist Dr Vinod Balasubramaniam, who is also a senior lecturer (microbiology) at the Jeffrey Cheah School of Medicine and Health Sciences in Monash University Malaysia, said an interval of between four and eight weeks between AstraZeneca jabs was preferred during in an outbreak situation.
In a non-outbreak setting, he said, the preferred interval remained at 12 weeks because early clinical trials (before the arrival of the Delta variant) found higher vaccine efficacy at protecting against symptoms.
"We are in an unprecedented outbreak situation where every second counts.
"The longer a person takes to be fully vaccinated, the higher the chances of the virus mutating and transmitting in the community.
"Unvaccinated individuals are the perfect reservoir for the virus to multiply and mutate.
"Protection against severe disease and death is crucial, and data show that the AstraZeneca vaccine offers strong protection against that, including when it comes to the Delta variant."
Dr Vinod said current strategies against Covid-19 should be tailor-made for the Delta strain as it was the most virulent and transmissible strain among all the other Variants of Concern, as well as the predominant strain.
The protection of vaccines against infection and hospitalisation with the emerging Delta have been studied internationally, where a dose of AstraZeneca reduces the risk of symptomatic infection by around 30 per cent and hospitalisation by 71 per cent.
However, two doses of the AstraZeneca vaccine reduce the risk of symptomatic infection even further, by 67 per cent, and the risk of hospitalisation by 92 per cent.
"Thus, shortening the gap between the first and second doses will bring short-term protection, which is expected to be beneficial in outbreak situations," said Dr Vinod.
On the administration of the third and booster doses, he said the third dose was meant for people who were unable to sustain or produce an adequate immune response to the first two shots in the series.
"For them, that third shot is not a booster but is a needed additional shot in their primary vaccination series to allow them to develop an effective immune response against a severe (Covid-19) infection and death.
"People who are less likely to mount an appropriate immune response are those on steroid-related treatments, who have received organ transplants, have severe HIV (human immunodeficiency virus) infections, or other conditions."
Booster shots, he said, were given to those who, while their bodies could produce the optimum response after two doses, experienced a drop in immune response after few months to a point where it could no longer provide adequate protection against the disease.
On concerns surrounding the Sinovac vaccine's efficacy, he said the government could allow private hospitals or clinics to administer booster shots with the option for the people to pay for it.
This, however, depended on whether substantial clinical data show that there was indeed severe waning of antibody and protection levels, as well as efficacy against Delta, he said.
A recent Health Ministry study revealed that high cases of breakthrough infection, leading to mortality, came from Sinovac recipients.