Only a few healthy adults need to get the fourth Covid-19 jab as it can provide only limited protection for one to two months, vaccine advisor Lau Yu-lung says.
His comments came after authorities allowed uninfected citizens aged 18 and 59 to get a fourth jab six months after their third shot.
But yesterday, Lau, who chairs the Department of Health's Scientific Committee on Vaccine Preventable Diseases, said only those who have high infection risks should get the fourth shot.
"They should take it only if they're going to travel overseas in the coming two months, such as visiting European countries or the United States, where mandatory masking has been lifted. Or they work in high- risk sites or front-line medical duties," he said on a radio program.
Lau said inoculation of the fourth jab remains a global controversy, as Hong Kong is the fourth jurisdiction worldwide to give it the green light, after Israel, the US and France.
He said if a person takes the same vaccine repeatedly, his body will recognize the jab and get used to it, lowering its effectiveness.
"It's like a student who revises the same material for the fourth time. He can only get 70 marks at most," he said.
Repeated vaccination may even bring adverse effects, including increasing the risk of developing a potentially fatal inflammation of heart muscles, which is more prevalent in teens after receiving the BioNTech jab.
Lau, who is also the chair professor of pediatrics at the University of Hong Kong, said local figures show one in 3,000 teens develop the condition after their second jab, five times more likely than before mass vaccination.
Although the risk of developing inflammation after the third jab is lower than the second, he believed the risk still stood after receiving the fourth shot.
"The fourth jab also only offers very limited protection that can only prevent one's infection for one to two months," Lau said. "It's not worth it for a healthy person to take the risks to get a fourth jab.
"For a triple-jabbed adult, the risks of developing serious illness or dying from Covid infection is close to zero," he said.
"Only one in hundreds of thousands die."
Lau said the SAR's priority is to continue to boost the three-dose coverage among the city's seniors above 80. They are the most vulnerable, but only 20 percent of the age group are triple-jabbed.
He said Hong Kong is in the midst of a "honeymoon period" at the end of the fifth wave, as social activities resume gradually.
"The government should step on it and raise the vaccine coverage before the six wave arrives. For elderly people aged 80 and above, at least 90 percent of them should have taken three jabs," he said. "If we're able to do that, citizens need not worry too much about serious illnesses and deaths, even when a sixth wave is triggered by a new variant."
He said the fatality rate is 1.2 percent for a triple-vaccinated elderly patient, and the risk can be further compressed to 0.5 percent after the fourth jab.
"So it's still necessary for elderly people above 60 and chronic disease patients to take the fourth jab for better protection," he said.
Immuno-compromised citizens including those with kidney problems should also get a fourth shot.
From April 8, seniors aged 59 and above have been allowed to receive a fourth jab three months after their third shot, regardless of whether they had taken the BioNTech or Sinovac vaccine.
However, the Food and Health Bureau said a fourth jab is not needed for those who have been infected after their third shot.
By Sunday, 6.68 million people, or 91.8 percent of the city's population aged three and above, had taken at least one jab, including 6.29 million, or 86.4 percent, double-jabbed.
About 3.61 million, or 53.3 percent, have taken the third shot and 97,733 people have got the fourth.
Lau's comments were echoed by another HKU infectious disease expert, Ho Pak-leung, who said three Covid jabs can effectively protect against severe illness.
"Authorities only started offering the third jab to all citizens in November, meaning not many people are eligible for the fourth jab now," he said. "There's no urgency for a healthy person to get a fourth shot now."
Earlier this month, US drug maker Moderna announced it is developing a jab against the fast-spreading Omicron variant which can be released in autumn at the earliest. Moderna is not available in Hong Kong.
