People should take it easy at home for at least half a day after receiving the second dose of a Covid-19 vaccine, an expert warned in saying it could have a more serious adverse reaction than a first jab.
This comes as the University of Hong Kong moves on a plan to recruit 100 people to take part in a 12-month clinical trial to check the effectiveness of taking one dose of the German-made BioNTech/Fosun vaccines and another of the China-made Sinovac to see whether that could deliver greater protection than taking both doses of one brand.
But HKU researcher Ivan Hung Fan-ngai, also the co-convener of the Expert Committee on Clinical Events Assessment Following Covid-19 Immunization, said he does not for now recommend anyone switching brands after a first shot.
Meantime, Chief Executive Carrie Lam Cheng Yuet-ngor, Chief Secretary Matthew Cheung Kin-chung, Executive Council members and all 23 pro-Beijing lawmakers will receive second doses of Sinovac 28 days after their first jabs on February 22.
Citizens will also start getting their second Sinovac jabs on Friday. That will also be 28 days after the SAR's inoculation program started on February 26.
So far, eight people have died following inoculations, including seven taking the Sinovac's CoronaVac shot and one man who received the BioNTech/Fosun Comirnaty vaccine.
On dangers, Hung said people who suffer from serious complications such as Guillain-Barre syndrome, facial paralysis and irregular cardiac rhythm after first jabs should not take a second one.
But people who had mild side effects such as dizziness and rash should be clear for a second jab, though they should seek advice from doctors.
Some of the side effects of the second shot, which serves as a booster, include redness and swelling at the injection site, fever, tiredness and muscle aches.
Respiratory expert Leung Chi-chiu said the second dose of the BioNTech/Fosun vaccine would result in greater side effects based on the manufacturer's phase three clinical trials.
"Design-wise, it uses mRNA technology," he said, which means a possibility "of a severe immune response."
He added: "For those who feel fine after the first jab the probability of having complications when having the second will be relatively lower. They need not feel anxious for having the booster shots."
Leung also said there is no evidence to suggest more severe complications for booster shots of the Sinovac vaccine or for the one from AstraZeneca vaccine. That British-made jab is Hong Kong's third vaccine option, with the first batch expected to arrive in late June.
Hung said the HKU clinical trial on "switching" Sinovac and BioNTech/Fosun vaccines, which will not start before next week, requires 100 healthy volunteers aged between 18 and 60.
The 100 will receive the BioNTech/Fosun vaccine as the first dose and Sinovac as the second 28 days later, and researchers will monitor their health for 12 months.
Hung also noted the vaccines use different platforms - inactive virus and mRNA - to stimulate the immune system to generate antibodies. The hypothesis is that taking one dose of each can generate greater protection than using one type for both shots.
Hung also said he understood people's concern about deaths following jabs, and the current mechanism allows people to postpone a second Sinovac inoculations for four weeks as the antibodies from the first should last for three to six months.
In Beijing, Sinovac spokesman Liu Peicheng said a small proportion of elderly people have suffered adverse effects following a vaccination, but that is similar to other adults.
So far 70 million Sinovac jabs have been administered worldwide in 30 countries, and in many the elderly have priority in receiving jabs.
The latest death in Hong Kong was of a 66-year-old man. He died on Friday, three days after receiving the first dose of the BioNTech/Fosun jab. That was the first death in Hong Kong of a recipient of that vaccine.
The man - a smoker - had a history of diabetes, hypertension and high blood fat.
Hung said his committee meets again on Wednesday to look into the death, adding that the panel has studied the seven fatalities of people who had Sinovac jabs and believed they were not linked directly to the vaccine. But final autopsy reports are awaited.
He said direct links to the inoculation may be established if a person experienced a severe allergic reaction or died after a jab while not having any serious chronic illness.
As of Saturday, around 330,600 people have received a first inoculation - 224,600 of the Sinovac jab and 106,000 of the BioNTech/Fosun vaccine.
Friday saw 12 people sent to hospital from vaccination centers. Seven had received BioNTech/Fosun shots and five the Sinovac jab.
One was a 57-year-old woman who received a BioNTech/Fosun jab at Tsuen King Circuit Sports Centre in Tsuen Wan. She experienced high blood pressure and is being treated at Yan Chai Hospital and is in a serious condition.