People in good health who have already had three Covid-19 boosters may not benefit from administering any more as they appear unlikely to guard against new strains of the virus, according to a local medical expert who cited the results of a recent US study.
`This is because existing Covid-19 vaccines are based on the Wuhan strain of the coronavirus, whereas the dominant new strains have mutated greatly, said Dr Thiravat Hemachudha, chief of the Thai Red Cross's Emerging Infectious Diseases Health Science Centre.
He was responding to questions about why Move Forward Party leader Pita Limjaroenrat -- who has reportedly had six Covid-19 vaccine shots -- recently contracted the virus for the third time.
"Since these vaccines aren't really effective in protecting against new infections, there is no need to keep injecting them repeatedly, Dr Thiravat said. "The mRNA vaccines, in particular, are associated with a heightened risk of heart inflammation."
The bivalent booster that is widely used in Thailand is only 4% effective in protecting against Covid-19 infections, he said, citing findings from a new US study conducted by Cleveland Clinic on more than 50,000 people.
"Worse yet, receiving more Covid-19 booster shots seems to be linked to a higher rate of new infections, a trend that is still not clearly understood," he said.
Virologist Yong Poovorawan described the rising caseloads as a seasonal epidemic that is expected to continue until August.
While the Omicron subvariant XBB.1.16 is the dominant strain for new infections in Thailand at present, a study conducted on 250 people in the country has shown that the severity of a second infection is less than the first time round, said Dr Yong, chief of the Centre of Excellence in Clinical Virology at Chulalongkorn University's Faculty of Medicine.
"So getting infected a third time [or more] isn't unusual. The Covid-19 virus has now become more like the influenza virus. The subvariants ... change constantly," he said.
The remaining immunity from previous Covid-19 infections or vaccinations does, however, provide some protection against severe illness and death from new infections, Dr Yong said.
The mutation rate of the virus has over the past year been observed to be closer to that of Type A influenza, he said.
Nonetheless, Dr Opas Karnkawinpong, permanent secretary for public health, insisted booster shots are still recommended for those with underlying medical conditions.
