By Meera Senthilingam, CNN

(CNN) -- One in every six cases of HIV in Europe is diagnosed in people over the age of 50, a new study has found.

Between 2004 and 2015, the number of new HIV diagnoses increased by 2.1% each year among this age group, with people over 50 accounting for 17.3% of new HIV cases diagnosed in Europe in 2015.

Experts argue sexual health programs should increasingly target this demographic, as well as the younger population.

"Our findings suggest a new direction in which the HIV epidemic is evolving," said Lara Tavoschi, a scientific officer at the European Center for Disease Prevention and Control (ECDC), who led the study published Tuesday in the medical journal Lancet HIV. "We see a steady increase in the number of new (HIV) diagnoses among older adults in the region."

The route of transmission was mostly heterosexual, Tavoschi confirmed.

"We need to increase awareness campaigns among older age groups," she told CNN.

Rise for some, fall for others

Using routine annual surveillance data from 31 countries, reported to the European Surveillance System between 2004 and 2015, the team at the ECDC analyzed new HIV diagnoses among people aged 15 and above.

The rate of HIV diagnosis among people over 50 increased in 16 countries, including Germany, Ireland and Belgium, and decreased in just one country, Portugal.

Rates were highest in Estonia, Latvia and Malta, where more than seven new cases were diagnosed per 100,000 older people by 2015. Numbers also increased among younger people in these countries, aged 15 to 49 years.

In certain countries, however, such as the United Kingdom and Norway, new diagnoses went down among young people, but increased in the over-50 population, with more than a 3.6% increase in newly diagnosed HIV cases each year in both of these nations.

"This is a result of successful awareness campaigns that may not have targeted older adults enough," Tavoschi said, speculating on one reason behind the trend.

England has a national HIV prevention program in place, for example, using local activities and social marketing to promote national HIV testing weeks and a campaign called "It starts with me" to increase testing and condom use, reduce stigma and inform people about sexually transmitted infections and practicing safe sex.

Previous studies have shown a stigma attached to older people having a sex life being at play, added Tavoschi, and the lack of sex assumed among this age group "is not a real reflection of what is happening in this group today," she said, preventing health care providers from discussing sexual health with older patients.

The data also showed that while diagnoses among men are rising among younger and older people across Europe, the numbers are decreasing among younger women, but increasing among older ones. For now, "it's unknown why," Tavoschi told CNN.

Last to know

In addition to rising rates, the study also discovered that people over 50 were also more likely to be diagnosed with advanced HIV disease -- meaning they will have been infected for longer without knowing.

The team at the ECDC explored levels of CD4 immune cells -- the cells typically damaged by HIV -- of more than 60% of people diagnosed in 2015 to identify the proportion of people diagnosed later into their infection, shown by lower levels of these cells.

Among all age groups, 47% of people analyzed were diagnosed late, but when divided out by age, 63% of people older than 50 were delayed in going to their clinic to be diagnosed, compared to 43% among those younger than 50.

"Being HIV positive and not knowing may endanger other people by transmitting the disease unknowingly," said Tavoschi, adding that some studies have also shown lower survivability among older people living with HIV.

"This study reminds us that we cannot afford complacency when it comes to HIV. People of all ages need to be reminded that the HIV epidemic is not over and viral transmission is still a very real possibility in all parts of the world in all populations and age groups," said Linda-Gail Bekker, president of the International AIDS Society, who was not involved in the research.

"Surveillance such as this is critical for us to keep a real and present focus on where resources and efforts need to go ... With an increased focus on young people, we need to be sure our messaging doesn't leave out other critical populations."

An evolving epidemic

While the new findings offer insight into a previously under-recognised demographic in terms of HIV control, Tavoschi acknowledges that in terms of the global HIV control, older people are not driving the epidemic. More than 36 million people were estimated to be living with HIV/AIDS globally in 2015.

While 2.5 people per 100,000 over the age of 50 were diagnosed with HIV in these 31 countries in 2015, 10.4 people per 100,000 under 50 were diagnosed that same year -- four times higher. "Clearly the magnitude of one group is bigger," said Tavoschi.

But, she points out, "the epidemic is evolving and going in a new direction that we need to take into account ... by health care providers."

"This issue is not something public health has given adequate attention to -- older people are 'at risk' but this is often not appreciated," said Janet Seeley, professor of anthropology and health at the London School of Hygiene & Tropical Medicine in the UK, who wrote a related commentary on the findings. She highlighted the importance of campaigns such as "Age is not a condom" in the UK.

"It highlights the need to use a condom because even if pregnancy is no longer likely, sexually transmitted infections including HIV, are."