The study shows there was a 64% reduction in targeted strains of HPV among teen girls following introduction of the vaccine.
Now, new research finds that in the 6 years following human papillomavirus (HPV) vaccine introduction, rates of infection of the four strains targeted by the vaccine have plummeted in teen girls, and they have also fallen significantly in other young women.
The study - from the Centers for Disease Control and Prevention (CDC) - is published in the journal Pediatrics.
The authors say the findings extend previous results about the impact of the HPV vaccine in the US and provide the first national evidence of its effect on women in their 20s.
The aim of the HPV vaccination campaign is to protect against cancer. More than 40 types of HPV can infect the genital areas of males and females. These types can also infect the mouth and throat. The virus is transmitted from one person to another during sexual activity.
HPV can cause cervical, vaginal and vulvar cancers in women and cancer of the penis in men. It can also cause anal cancer, cancer of the back of the throat (oropharynx), and genital warts in both men and women.
HPV is very common: the CDC estimate that currently in the US, there are around 80 million people infected with HPV, with 14 million new infections - including among teenagers - occurring every year.
Most people infected with HPV do not know they have it.
The HPV vaccine is recommended for pre-teen males and females aged 11-12 so that they are protected before there is a chance of exposure. The vaccine also produces a more robust immune response if received at this age.
Another reason to give the vaccine to pre-teens is because later on, they are less likely to get regular health checks, thus providing fewer opportunities to receive the vaccine routinely.
HPV vaccine is given in three shots - with the second shot 1 or 2 months after the first, and the third 6 months after the first.
64% reduction in teen girls
For the study, researchers looked for genetic evidence of HPV in cervical/vaginal samples that were self-collected by females aged 14-34 years who took part in National Health and Nutrition Examination Surveys.
They compared data from 2,587 females whose samples were collected before the vaccine was introduced (2003-2006) with those of 2,061 females whose samples were collected afterward (2009-2012).
The results showed that the percentage of females who reported receiving at least one dose of HPV vaccine following its introduction ranged from 3.3% of those aged 30-34 years to 51% of those aged 14-19 years.
They also show there was a 64% reduction in prevalence of the four strains of HPV included in the vaccine in teen girls aged 14-19 years and a 34% decrease among young women aged 20-24 years.
The HPV vaccine immunizes against HPV types 6, 11, 16 and 18, the strains thought to cause the most cancer cases. The researchers note that:
"There were no statistically significant changes in other HPV type categories that indicate cross-protection."
In October 2015, Medical News Today learned that the reason some parents are not having their children vaccinated against HPV may be largely down to discouragement from doctors.