SINGAPORE: Extending the time between COVID-19 vaccine shots could allow more people to be protected earlier and save lives, health experts in Singapore said.
With the first jab already offering good protection against COVID-19, people can “quite safely wait a little longer” for their second jab, the experts told CNA.
One study suggests that this strategy could help reduce infections and deaths if certain conditions are met. A separate study also found that delaying the second jab could boost immune response in some people.
This comes after Health Minister Ong Ye Kung said on Sunday (May 16) that Singapore is studying if it should extend the time between vaccine shots to allow more people to get protection from a first dose.
Facing a limited vaccine supply, the United Kingdom at the end of last year announced that it would delay the second dose by up to 12 weeks after the first dose to maximise the number of people who would at least be partially protected.
So far, Singapore has followed the regime recommended by vaccine manufacturers – the two doses of the Pfizer-BioNTech vaccine are given three weeks apart and the two doses of the Moderna vaccine are given four weeks apart.
International studies have shown that one dose of the vaccine offers "good protection", Mr Ong said, adding that the time between the two doses can be extended to between six weeks and eight weeks without "materially impacting the efficacy of the vaccine".
If the new strategy is implemented, it will take place in phase two of Singapore's vaccination programme.
The first phase of the national vaccine programme has been focused on those who "need it most", Mr Ong said. This includes seniors, who are more vulnerable to severe disease, as well as frontline and essential workers.
Singapore is now approaching phase two of its vaccination exercise, Mr Ong added, with the Ministry of Health (MOH) studying how to proceed.
Professor Dale Fisher, senior consultant at the National University Hospital’s division of infectious disease, said increasing the interval between doses is a “good strategy” that is “strongly” supported by scientific studies.
“Even extending (the interval) to 12 weeks suggests ultimately a stronger immune response. At the same time we can get more people vaccinated earlier,” he said.
Professor Paul Tambyah, president of the Asia Pacific Society of Clinical Microbiology and Infection, said the new strategy could potentially allow Singapore to vaccinate twice as many people while waiting for new supplies.
A larger pool of vaccinated people could be crucial as Singapore battles different variants of the virus, said Associate Professor Hsu Li Yang, Vice Dean (Global Health) at the National University of Singapore's Saw Swee Hock School of Public Health.
“If by chance we are not able to contain the spread of B1617 or other future emerging variants, then having a larger proportion of the population vaccinated will save lives and may help reduce the size of the outbreak,” he said.
According to a US predictive modelling study, delaying the second dose among people younger than 65 could lead to fewer people dying of the disease, but only if certain conditions are met, Reuters reported.
The specific conditions include having a vaccine with a one-dose efficacy of at least 80 per cent, and having daily immunisation rates of between 0.1 per cent and 0.3 per cent of a population.
The study, published in the BMR British medical journal, used a simulation model based on a "real-world" sample of 100,000 US adults and ran a series of scenarios to forecast potentially infectious interactions under different conditions.
Assoc Prof Hsu said the difference in efficacy between having one and two jabs is about 10 per cent to 20 per cent, based on different studies, although he noted that having “less protection is better than no protection”.
Nevertheless, Prof Tambyah said there is a risk that incompletely protected individuals could lead to "true vaccine escape mutant strains or variants" that develop in response to the vaccines.
"This has happened with Hepatitis B, with strains of Hepatitis B that can escape current vaccines," he said. "These tend to be less virulent but we do not know if this will be the case with SARS-CoV-2."
SARS-CoV-2 is the virus that causes COVID-19.
DELAYING SECOND JAB COULD BOOST ANTIBODY RESPONSE
Despite that, a separate UK study showed that in those older than 80 years, delaying the second dose of the Pfizer–BioNTech vaccine could boost antibody responses by more than three times after the second jab.
Researchers studied 175 vaccine recipients older than 80 years who received their second dose of the Pfizer-BioNTech vaccine either three weeks or 11 weeks to 12 weeks after the first dose.
The team measured recipients’ levels of antibodies against the SARS-CoV-2 spike protein and found that peak antibody levels were 3.5 times higher in those who waited longer for the second dose.
While Assoc Prof Hsu acknowledged that this study has not been published in a peer-reviewed journal yet, he said the results are “promising” and can likely be safely extrapolated to younger individuals.
“No doubt this study and those that showed high vaccine efficacy after a single dose led our MOH to consider a possible alternative strategy,” he said.
Despite that, Associate Professor Richard Sugrue at the Nanyang Technological University’s School of Biological Sciences warned that vaccine efficacy at the population level is still not completely understood.
For instance, Assoc Prof Sugrue questioned how delaying the second jab could affect the likelihood of fully vaccinated individuals spreading the virus, which has been seen in Singapore.
“If this can be demonstrated to be effective in a sufficient number of people in Singapore, this may convince the authorities (to implement the new strategy),” he said.
HOW WILL VACCINE SUPPLY AFFECT THE NEW STRATEGY?
Assoc Prof Hsu said a limited supply of COVID-19 vaccines and a rapid spread of infections could also push authorities towards rolling out the new strategy.
“If supplies were truly constrained, which Health Minister Ong Ye Kung has assured is not the case here, then many people could be left with having received only a single dose of vaccine for a prolonged period of time beyond the 12-week mark,” he added.
“But other than the individual perception of incomplete vaccination, at a population level, this is not a bad thing.”
In response to questions on whether a tight supply of COVID-19 vaccines has slowed the pace of vaccination in Singapore, Mr Ong said that the pace is limited by how quickly vaccines arrive in the country.
Prof Fisher believes Singapore is buying as many of the approved vaccines as it possibly can, pointing out that the new strategy is about giving a dose to a person for a first jab, instead of someone else as a second jab.
"Put simply, we want as many people as possible to get their first jabs. Those needing a second jab can quite safely wait a little longer," he said.
Mr Ong said on Sunday that those who already have an appointment for a second jab would not be affected by the new strategy.
While Prof Fisher said communicating the changes in vaccine intervals to people could prove to be a “headache”, he reiterated that the change would lead to decreased transmission and less severe disease.
“We have to move with the evidence,” he stated, citing the studies done on delaying the second jab. “It’s a rapidly changing field. Disease prevalence also influences the timing by which a country adopts new practices.”
Prof Tambyah said it would be good to keep an eye on the data coming out of the UK's strategy, especially as the country has "very good" monitoring of the molecular characteristics of the circulating viruses.
"Of course, it would be ideal if we have enough vaccines but until then, we can see what happens in the UK," he added.