Tighter investigation and research of every cause of new SARS cases is vital for China to contain the epidemic more effectively, experts said Thursday.

Except for North China's Hebei Province, other areas of the Chinese mainland have seen a continuously slow decrease in both confirmed cases and suspected ones, said Nie Jun, an official from the Epidemic Analysis Group of the National Task Force for SARS Prevention and Control.

But experts are still unclear on how the virus is spread with the cause of many newly reported cases still unknown, Xu Dezhong, an expert with the group, told China Daily in an exclusive interview.

In the 24 hours to 10 am Thursday, the Chinese mainland reported 52 new SARS cases, four deaths and 86 suspected cases, the Ministry of Health said Thursday.

It is for the sixth straight day that the Chinese mainland has seen fewer than 100 new SARS cases.

The cumulative numbers of SARS cases, deaths and suspected cases rose to 5,163, 271, and 2,278 respectively.

Beijing reported 27 new SARS cases Thursday - 12 fewer than that reported in the previous day.

According to Beijing health authority, about 40 to 50 per cent of the newly reported cases in the city are people who had been isolated after coming into close contacts with SARS patients.

Visiting experts from the World Health Organization have said it is too early for China to say the epidemic has been contained.

Wang noted new cases may include imported ones, people infected by SARS patients without being isolated, and even those infected by people who may carry virus but do not show any symptoms, which is also called "recessive spreading.''

Recessive spreading is possible although no evidence has been found to prove it, Wang said. "Generally speaking, the virus spreads from people who have fallen ill to others through close contact.''

Chinese experts, such as those from South China's Guangdong Province, intend to research recessive spreading, Wang said.

It has been proved the virus is spread by people breathing in flying particles during close contact with others. Whether the virus can become airborne or not needs further research, Wang said.

He also dismissed the notion that some new patients may have been infected by the virus from other sources, such as animals.

It is possible that in Guangdong - widely held to be the origin of SARS - the virus was passed to humans from animals.

But this is extremely unlikely in other regions. One reason is that the virus can now lead an active life and spread quickly among humans, Wang said.

Official statistics show that Beijing has not found any animal infected with or spreading the virus.

Health authorities are not able to find the causes of all SARS cases and reasons include the fact that registered SARS patients do not wish to talk about their experiences. This potentially puts some infection sources beyond the control of the epidemic surveillance and investigation system, Wang noted.

Editor: Liao Ming