Though initially contracted intravenously, the virus is also thought to have spread into the wider community by means of heterosexual sex.
Preliminary analyses suggest that the strain in question is a genetic variant known as CRF01-AE.
A total of around 25 people are so far reported to have been infected, according to the Swedish Institute for Infectious Disease Control (Smittskyddsinstitutet - SMI).
There are no indications that the current strain is particularly contagious or aggressive. But SMI is recommending extra vigilance and an increase in preventive measures, such as providing easier access to testing for intravenous drug users and their contacts in Stockholm.