A study of the health and inoculation records of millions of Hong Kong residents has found three shots of a Covid-19 vaccine are effective in preventing death and serious complications among the elderly and people with chronic illnesses.

But researchers at the faculty of medicine at the University of Hong Kong (HKU) on Tuesday said additional data was needed to assess what effect additional jabs beyond the first three had in affording the groups the same benefits.

Analysing the health and vaccination records of more than 4 million people maintained by the Hospital Authority and Department of Health, researchers determined three Covid-19 shots reduced deaths among infected people aged 60 and above by 80 per cent.

Three jabs also reduced by the same proportion the rate of hospitalisation and incidence of severe complications among the same age group.

Just over 70 per cent of people aged 80 and above have received their first dose, while almost 65 per cent of them have received three jabs.

The study found three vaccine doses could reduce Covid-19 related deaths for people with multiple chronic illnesses by 90 per cent and by 95 per cent for those aged 80 and above.

Professor Ian Wong Chi-kei, head of the department of pharmacology and pharmacy, said it was necessary to use a large data set to assess the effectiveness of the vaccines in Hong Kong.

“Vaccination can reduce mortality, hospitalisation [and] serious complications, specifically for older people,” he said. “From our studies, we hardly found any serious side effects [among the elderly and chronically ill] at all. That information is consistent with other clinical trials overseas. The risk is minimal but the benefit is huge.”

Wong called the decision to take Covid-19 shots a “no-brainer”.

He also explained data obtained from clinical trials in a controlled study might not fully reflect the real-world situation, hence the need for such a large-scale analysis.

Large-scale studies of people who had taken the German-made BioNTech shots mainly focused on the non-Chinese population, while research on the effectiveness around the world of the Sinovac doses manufactured in mainland China was also limited, he added.

Further study was needed to precisely determine how long immunity from booster jabs lasted and the effectiveness of shots beyond the first three, Wong said.

Dr Francisco Lai Tsz-tsun, a research assistant professor at the pharmacology department, said the data used in their study had been anonymised.

Lai also said he believed that the elderly or those with chronic diseases did not have enough information to decide whether they should take the vaccine or not.

“They do not know how effective the vaccine is to prevent Covid-19 … and [the] safety of the vaccines. But now we have used electronic health records to look at vaccine effectiveness from millions of people and tens of millions of medical records,” Lai said. “This will be good reassurance to people who haven’t received the vaccine.”

The vaccination rate among the elderly remains low. Just over 70 per cent of people aged 80 and above have received their first dose, while almost 65 per cent of them have received three jabs.

By comparison, almost 95 per cent of the general population has received a first dose, while almost 84 per cent have taken three.

The city was hit by the fifth wave of coronavirus infections last year, with most cases logged from January to April.

More than 95 per cent of the 13,400 coronavirus-linked deaths recorded since the pandemic hit happened during the fifth wave.