STOCKHOLM (Reuters) – Sweden’s hopes of getting help from herd immunity in combating the coronavirus received a fresh blow on Thursday, when a new study showed fewer than anticipated had developed antibodies.

Sweden’s has opted for a more liberal strategy during the pandemic, keeping most schools, restaurants, bars and businesses open as much of Europe hunkered down behind closed doors.

While Health Agency officials have stressed so-called herd immunity is not a goal in itself, it has also said the strategy is only to slow the virus enough for health services to cope, not suppress it altogether.

However, the study, the most comprehensive in Sweden yet, showed only around 6.1% of Swedes had developed antibodies, well below levels deemed enough to achieve even partial herd immunity.

“The spread is lower than we have thought but not a lot lower,” Chief Epidemiologist Anders Tegnell told a news conference, adding that the virus spread in clusters and was not behaving like prior diseases.

“We have different levels of immunity on different parts of the population at this stage, from 4 to 5% to 20 to 25%,” he said.

Herd immunity, where enough people in a population have developed immunity to an infection to be able to effectively stop that disease from spreading, is untested for the novel coronavirus and the extent and duration of immunity among recovered patients is equally uncertain as well.

Sweden surpassed 5,000 deaths from the coronavirus on Wednesday, many times higher per capita than its Nordic neighbors but also lower than some countries that opted for strict lockdowns, such as Britain, Spain and Italy.