"It is very unusual for us to see this type of outbreak in Sweden," acting state epidemiologist Erik Sturegård told the TT news agency.
All known cases are directly linked to the festival, which took place from July 30th to August 2nd in Sollefteå municipality. A majority of those infected are children under the age of 18.
"It is an illness that can cause very severe disease, even in otherwise healthy children," said Sturegård.
Cases have been reported from the regions of Jämtland Härjedalen, Skåne, Stockholm, Västerbotten, Gotland, Värmland and now also Västernorrland – which previously had no confirmed cases despite hosting the festival.
A major widespread outbreak is unlikely in Sweden, which boasts a 95 percent immunisation coverage. The vaccine is included in Sweden’s national childhood vaccination programme.
In most of the reported cases, the individuals were not vaccinated against the virus.
"Even though we have 95 percent vaccination coverage in the childhood vaccination programme, there are small pockets of people who, for various reasons, have not been reached by the vaccination drive or have chosen not to get vaccinated. Because measles is so extremely contagious, a small local outbreak can happen very easily," said Sturegård.
Privately-owned vaccine clinic chain Vaccin Direkt noted that the outbreak has triggered a sharp rise in the number of people seeking vaccination, reporting a 450 percent increase in administered doses compared to the same period last year. Those who have got vaccinated in recent days are mainly people born in the 1970s or earlier who have not received two doses through the national immunisation programme, but younger people are also booking appointments.
"People born after 1980 who were actually offered two doses of the measles vaccine through the national programme, but who for various reasons haven’t completed these vaccinations," Charlotta Zacharias, a doctor at Vaccin Direkt, told TV4 Nyheterna.