COPENHAGEN, Denmark—Swedish authorities said Thursday that citizens from fellow Nordic countries will have to show a valid COVID-19 vaccination certificate when entering Sweden starting next week. As of Dec. 21, people from Norway, Finland, Denmark, and Iceland no longer will have an exemption to the certificate requirement and must also show their passes to enter Sweden. Swedish Social Affairs Minister Lena Hallengren also encouraged all travelers to be tested for the coronavirus upon entry due to a “deteriorating” public health situation. “The spread of infection is increasing sharply. The new virus variant omicron makes it difficult to predict the spread of infection in the future,” Hallengren said during a news conference. Sweden has previously stood out among European nations for its comparatively hands-off response to the coronavirus. The Swedish government has not ordered lockdowns or closed businesses during the pandemic, relying instead on citizens’ sense of civic duty to control …