Mix and match: Clinics see some demand for Sinovac, Sinopharm as COVID-19 booster among people who got mRNA vaccine
SINGAPORE: Some people are already mixing different types of COVID-19 vaccines by taking the Sinovac and Sinopharm non-mRNA jabs as booster shots, even as authorities are still studying such a strategy, which is known in medical terminology as a heterologous approach.
Singapore on Oct 9 expanded its booster programme to include healthcare workers, frontline workers and those aged 30 and above, but only using the Pfizer or Moderna mRNA vaccines, which were also administered under the national vaccination programme.
But some have paid to get the Sinovac and Sinopharm vaccines, offered under the Special Access Route, as boosters after they were previously double-jabbed with an mRNA vaccine, according to two clinics CNA spoke to.
Both the Sinovac and Sinopharm vaccines are inactivated vaccines.
These people got their second jab about three to six months before and wanted to get an immunity boost ahead of the national booster programme, or were wary of severe side effects they had experienced with the mRNA vaccine, the clinics said.
While there is currently limited data on the effectiveness of a heterologous booster strategy, the pre-print of a US study on heterologous booster vaccinations posted on Oct 13 stated that both "homologous and heterologous booster vaccinations were well-tolerated and immunogenic in adults who completed a primary COVID-19 vaccine regimen at least 12 weeks earlier".
Immunogenicity refers to the vaccine's ability to produce an immune response.
The researchers also wrote that the reactogenicity, or the vaccine's ability to produce common adverse reactions, "did not differ" between heterologous and homologous boosters.
The Ministry of Health (MOH) did not respond to requests for comment for this article.
MOH director of medical services Kenneth Mak said during a press conference on Sep 17 that mRNA vaccines under Singapore's booster programme could be "treated as equivalent to each other", meaning people could opt for a Moderna booster if they had taken two doses of the Pfizer jab, or vice-versa.
This comes at the advice of the Expert Committee on COVID-19 Vaccination, which he said was also studying the mixing of vaccines and was expected to make a recommendation on the issue "very soon".
Dr Chua Hshan Cher, medical director of the Phoenix Medical Group which offers the Sinovac vaccine, said those who took the jab as a booster only formed a "minority" of his customers.
"We explain that there is no data to support their choice, and that the risks of mixing vaccines cannot be definitively elucidated," he told CNA on Oct 4, when the clinic had administered more than 1,200 Sinovac doses, mostly to the unvaccinated.
"However, based on our experience of the past 10 days, it appears that the vaccine is very well tolerated and no major adverse reactions have been reported."
Dr Chua said some of the patients who opted for a heterologous booster had cited side effects, like fever and headaches not amounting to allergies, with the mRNA jabs, and did not want to experience those side effects again.
"Another group of patients cited the need to travel to China at some point and hence preferred the convenience Sinovac would offer them during their travel application process," he said.
While Dr Chua acknowledged there was no verified published data at that point on using the Sinovac jab as a booster, he pointed to a pre-print study posted on Sep 5 that indicated a third shot of an inactivated vaccine "produces a highly sifted humoral immune response via a sustained evolution of antibodies capable of effectively neutralising SARS-CoV-2 variants of concern".
Dr Chua said the study showed that a third shot of the Sinovac vaccine "given six months or more after the second shot would result in a remarkable increase in antibody level".
"We will have to wait for more information as more rigorous studies are carried out with regard to heterologous vaccine boosting strategies," he said.
When asked about the potential risks associated with mixing vaccines, Dr Chua said there is "insufficient data currently to conclusively determine the risk".
Dr Leong Hoe Nam of the Rophi Clinic, which offers the Sinopharm jabs, likewise told CNA on Oct 1 that a heterologous booster strategy is "not proven to have no risks or have benefits".
"Potentially there may be a benefit. One of the antibodies generated by the spike protein will be able to deal with the (coronavirus) mutants by using a cross or what they call heterologous boosting," he said.
Dr Leong said he does not keep count of the number of people who have taken the Sinopharm jabs as boosters, noting that his clinic only started offering the jabs that week.
The infectious disease specialist said he only offers it to customers who have had their second jab at least three months before, although "anecdotal stories" show that a gap of six months would produce a better immune response.
Dr Leong said the Sinopharm vaccine is one example of a "mild" vaccine that is sufficient to act as a booster, adding that his clinic's aim is to cater to those who need a Sinopharm booster for various reasons including travel and are unable to get it from Government sources.
"People see the benefit of not falling sick. Think of the inconvenience to self, work and family as well as the social stigma," he said.
"If there was no restriction on vaccine use, individuals would, like the hepatitis B vaccine, keep the antibody levels high. But because it is restricted, they are not allowed to have their own choice."