An experimental birth control vaccine for men can effectively prevent pregnancy in their female partners by lowering sperm count, scientists including those from India have found.
Researchers are working to perfect the combination of hormonal contraceptives to reduce the risk of mild to moderate side effects, including depression and other mood disorders.
"The study found it is possible to have a hormonal contraceptive for men that reduces the risk of unplanned pregnancies in the partners of men who use it," said Mario Philip Reyes Festin, from the World Health Organisation (WHO) in Switzerland.
"Our findings confirmed the efficacy of this contraceptive method previously seen in small studies," said Festin.
Researchers, including Man Mohan Misro of the National Institute of Health and Family Welfare in New Delhi, tested the safety and effectiveness of injectable contraceptives in 320 healthy men ages 18 to 45.
The participants had all been in monogamous relationships with female partners between the ages of 18 and 38 for at least a year. The men underwent testing to ensure they had a normal sperm count at the start of the study.
The men received injections of 200 milligrams of a long-acting progestogen called norethisterone enanthate (NET-EN) and 1,000 milligrams of a long-acting androgen called testosterone undecanoate (TU) for up to 26 weeks to suppress their sperm counts.
Healthcare professionals gave the men two injections every eight weeks.
Participants initially provided semen samples after eight and 12 weeks in the suppression phase and then every 2 weeks until they met the criteria for the next phase.
During this time, the couples were instructed to use other non-hormonal birth control methods.
Once a participant's sperm count was lowered to less than one million/ml in two consecutive tests, the couple was asked to rely on the injections for birth control.
During this period known as the efficacy phase of the study, the men continued to receive injections every eight weeks for up to 56 weeks.
Participants provided semen samples every eight weeks to ensure their sperm counts stayed low. Once the participants stopped receiving the injections, they were monitored to see how quickly their sperm counts recovered.
The hormones were effective in reducing the sperm count to one million/ml or less within 24 weeks in 274 of the participants. The contraceptive method was effective in nearly 96 per cent of continuing users.
Only four pregnancies occurred among the men's partners during the efficacy phase of the study.
Researchers stopped enrolling new participants in the study in 2011 due to the rate of adverse events, particularly depression and other mood disorders, reported by the participants. The men reported side effects including injection site pain, muscle pain, increased libido and acne.
The study was published in the Journal of Clinical Endocrinology and Metabolism.
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