In case of suspected pregnancy, a single injection of this contraceptive will lead to pregnancy termination in 100 per cent cases and it is safe, Talwar said in his presidential address to the International Society of Reproductive Immunology meeting here today.

Talwar who was presented the society's life-time award for his research on immunology described this approach as ``passive immunization with preformed antibodies'' as opposed to active immunization with a vaccine which relies on antibodies made inside the body.

The birth control vaccine developed earlier by Talwar works by generating antibodies that neutralize the key pregnancy hormone called human chorionic gonadotrophin (HCG).

Although trials on women in India and elsewhere had shown it is safe, the vaccine worked only in 80 per cent women -- other failed to produce enoughanti-HCG antibodies to knock out the hormone. Its another drawback was that it had to be given to women two or three times a year.

Talwar told the international gathering that his quest to overcome the vaccine's drawbacks led him to the alternative approach of injecting preformed anti-HCG antibodies in sufficient quantity to ensure 100 per cent success.

``The effect would be quick, fully reversible and without side effects,'' he said.

Existing post-coital contraceptives have side effects and must be administered with a specified window period. But the anti-HCG antibodies can be administered even after several days, he said.

Talwar said his work to develop the human recombinant antibodies for the emergency contraceptive was being supported by grants from the Rockefeller Foundation of the United States.