Beijing, Oct 15 (IANS): China's CoronaVac and Sinopharm Covid vaccines may be waning in immunity levels, several studies have shown.

CoronaVac and Sinopharm -- both inactivated vaccines, which use killed SARS-CoV-2 virus -- account for almost 50 per cent of the 7.3 billion Covid-19 vaccine doses delivered globally. China administered about 2.4 billion doses of the vaccines to its citizens, but almost one billion doses have gone to 110 other countries, particularly the less wealthy nations, Nature reported.

However, many countries, including Seychelles and Indonesia, which used the vaccines reported Covid-19 surges earlier this year, sparking a debate about their waning protection and the need for boosters.

"These are not bad vaccines. They're just vaccines that haven't been optimised yet," Gagandeep Kang, a virologist at the Christian Medical College in India's Vellore, who advises SAGE, was quoted as saying.

After receiving a second dose of CoronaVac, only 60 per cent had high levels of neutralising antibodies one month, compared to with 86 per cent of those who had received two shots of the Oxford-AstraZeneca vaccine, revealed a study of 185 health-care workers in Thailand, not yet peer-reviewed.

After three months, the antibody prevalence dropped to just 12 per cent in the same group, Opass Putcharoen, an infectious-diseases specialist at the Thai Red Cross Emerging Infectious Diseases Clinical Center in Bangkok, was quoted as saying.

However, "waning of antibodies isn't necessarily the same as waning of immune protection", noted Ben Cowling, an epidemiologist at the University of Hong Kong.

CoronaVac induces a significantly lower antibody response compared with Pfizer-BioNTech's mRNA jab one month after two doses, however, the T-cell response was comparable, showed a study from Hong Kong, also not peer-reviewed.

Vaccines made using other technologies have seen a similar trend of waning antibodies and protection against infection, but more-robust protection against severe disease and death. But researchers say that because the Chinese inactivated vaccines start at a lower base of neutralising antibodies, the protection they offer could drop faster than those with a stronger head start, the Nature report said.

However, a drop in protection can prove deadly for the elderly. An analysis of about one million people, hospitalised with Covid-19 in Brazil, showed that CoronaVac offered up to 60 per cent protection against severe disease up to the age of 79. But in people over 80, CoronaVac was only 30 per cent effective at preventing severe disease and 45 per cent effective against death.

As a result, several countries, including Chile, Abu Dhabi in the United Arab Emirates and China, are giving booster jabs of mRNA or viral-vector vaccine to people who received the CoronaVac or Sinopharm vaccines, the report said.