Thousands of schoolgirls are to be vaccinated against the sexually transmitted virus that causes most cervical cancer cases as part of a national scheme starting this month, the NHS said on Monday. All girls aged 12 or 13 will be offered the jab at school, while older teenagers will take part in a catch-up programme over the next year. The NHS says the vaccine is most effective if it is given when girls are aged 12 or 13, before they start having sex. Critics have described the injection as a “sex jab” that may encourage under-age sex and promiscuity. However, Public Health Minister Dawn Primarolo said the routine vaccination of teenage girls against the human papillomavirus (HPV) is a crucial step that will save up to 400 lives a year. Girls will need three doses of the drug over six months. Those aged 16 or over do not need their parents' consent. The vaccination programme for 12- to 13-year-olds will include around 300,000 girls across England each year. By July 2011, including older girls vaccinated as part of the catch-up programme, over two million girls One academic said the vaccination should also be offered to boys because they contribute to the transmission of the disease.