Scientists say they have created the first birth-control shot for women, one that is effective for an entire year with a single injection.
Preliminary tests by scientists at the New Delhi-based National Institute of Immunology suggest the injection may prevent the fertilized egg from sticking to the wall of the uterus.
It is the first birth-control method that makes use of a woman's immune system. All other methods either physically block conception _ such as condoms, diaphragms, IUDs and sterilization _ or use steroids and hormones that are foreign to the body _ such as the pill or Norplant.
The inventors, who have conducted clinical tests for seven years, believe their shot is safe, effective and reversible. But it will be several years before tests are completed and it can be sold in India.
"We have proved that the vaccine does indeed protect women without any side effects. That's a very important milestone," said Dr. Gursaran Prasad Talwar, a leading immunologist conducting the project.
Talwar's team was spurred on by India's desperate need for an answer to rapid population growth, seen as the biggest hurdle to the nation's emergence from poverty.
Scientists in several countries, including the United States, also are trying to create a birth-control injection but none has claimed success equal to that announced by Talwar.
The injection has been tried successfully on 174 women in India, said institute spokesman Suresh Chandran. It starts wearing off after one year, when a booster shot is needed.
The main element of the injection is beta-hCG, which triggers the production of antibodies to hCG, or human chorionic gonadotropin, a hormone essential in sustaining pregnancy, scientists said.
Once the antibodies neutralize hCG, they said, which is produced only after fertilization, the fertilized egg is unable to stick to the uterine wall and is ejected in regular menstruation.
But Dr. Rosemarie Thau of the New York-based Population Council said it was not known precisely how the contraceptive works and suggested it might even prevent fertilization of the egg. If true, that could alleviate the concerns of anti-abortion activists, many of whom oppose contraceptives that act after the egg is fertilized.
Such concerns have provoked opposition to the so-called French abortion pill, as well as the IUD. Even when used as a morning-after contraceptive, as reported earlier this week, the abortion pill acts after the egg has been fertilized.
Talwar's team found that beta-hCG can be produced in several ways, including extracting it from pregnant women's urine or through genetic engineering.