Health officials in Hong Kong are concerned about the rise hepatitis A in men who have sex with men, and cite sexual transmission as behind the rise.

The Centre for Health Protection said 11 new cases have emerged since September 2015 and there were 24 people infected in the city. All of them have now recovered after treatment.

Of the 24, 20 of those are HIV positive.

Between 2006 and 2015, the city recorded zero to two cases of the viral infection per year.

“The latest situation suggests that, apart from spreading among HIV-positive men who have sex with men, the hepatitis A virus may have spread among [gay men] in the community by sexual transmission,” the centre’s spokesman said, according to the South China Morning Post.

Since Wednesday last week, free hepatitis A vaccinations have been given to gay men at public clinics in an attempt to reduce infections.

“Vaccinating susceptible individuals together with safer sex and strict personal hygiene are key to preventing hepatitis A via sexual transmission,” said the spokesperson.