As The Local reported earlier on Thursday, Löfven said in a Facebook post that he was isolating with his wife at home and would get tested soon after a person within his social circle tested positive for Covid-19.
“It's the only responsible thing to do in this situation,” Löfven said.
The head of government also warned that developments were “going in the wrong direction quickly.”
“More people are infected. More people are dying. It is a serious situation,” he said.
Löfven said he had not been in direct contact with someone confirmed to have the coronavirus but someone in his “vicinity” had.
Even though that person had since tested negative, he had decided to self-isolate on the advice of his doctor.
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After seeing a heavy death toll from March to June – over 5,000 in a country of 10.3 million inhabitants – Sweden registered a decrease in both cases and fatalities between July and mid-October.
Since then, the number of cases has soared and deaths have also started climbing in recent days.
On Thursday, the country reported another five deaths linked to Covid-19, bringing the total to 6,002.
“Looking at the number of deceased per week, we are also seeing an impact with increases over the last two weeks,” Karin Tegmark Wisell, head of microbiology at the Public Health Agency, told a press conference.
In response to the surge, Sweden, which has elected to curb the virus with mostly non-coercive measures, has issued stricter recommendations for heavily hit areas.
These usually include calls for people to avoid crowded indoor environments and if possible all physical contact with other than people in your own household.
On Thursday, these recommendations were extended to two more regions meaning more than half of Sweden's 21 healthcare districts have now received stricter local recommendations.
Another 4,034 cases was also reported in the last 24 hours, one of the highest daily totals on record, bringing the total to 141,764 cases.
In neighbouring Denmark, which is also facing a resurgence in cases, Prime Minister Mette Frederiksen is also in self-isolation after the justice minister tested positive.
An initial test proved negative, but her cabinet said on Wednesday evening that she would remain in isolation pending a second test.
The Public Health Agency counts Sweden's official death toll as the number of people who die within 30 days of testing positive for coronavirus, regardless of whether or not the virus was a cause of death. This is the figure that is usually used whenever coronavirus deaths in Sweden are talked about, and is presented Tuesday-Friday.
Sweden's National Board of Health and Welfare also keeps statistics, which instead count the deaths where a doctor determined that coronavirus was the cause of death, regardless of whether or not the person had tested positive as confirmed by laboratory analysis. The latest available figure stood at 6,034 on November 2nd – out of these, 90 percent or 5,424 had also tested positive according to the Public Health Agency's database.