Esther Chow Oi-wah, assistant professor of social work at City University who conducted the survey, expressed concern about what she described as "the hidden elderly" who had not been reached by the government's campaign.

Launched on October 19, the pneumococcal vaccine is provided free to elderly living in residential care homes, those who are chronically ill , followed by those attending in public clinics and receiving Comprehensive Social Security Assistance.

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They are entitled to a government subsidy of HK$190 per dose of the vaccine if they go to private doctors who have signed up for the scheme.

The survey of 295 elderly people between November 9-13 found that 52 percent had not yet been immunized.

Of those who have not received the vaccines, 48 percent said they did not plan to do so in the next three months.

Chow, who conducted the survey for welfare agancy St James Settlement, said even the active elderly population were reluctant to get vaccinated due to misunderstanding about the jab.

The number of people aged 65 and older is 900,000, according to the government's latest figures.

"They think only ailing people and young children would need the vaccines. Some even think pneumococcal diseases are mild and non-fatal. Many thought they do not need the vaccine as long as they can maintain good health," said Queenie Yim Fung-hi, manager of St James Settlement's health services.

In response, the Department of Health said it had not received any reports of elderly feeling unwell after receiving the vaccine. As of November 15, more than 57,000 people 65 and older have received the vaccinations under the subsidy scheme.

The World Health Organization has warned pneumococcal diseases are a major public health problem worldwide.

Pneumonia, febrile bacteremia and meningitis are the most common manifestations of invasive pneumococcal disease, whereas bacteria spread within the respiratory tract may result in middle- ear infection, sinusitis or recurrent bronchitis, according to the WHO.