RIVIERE-DU-LOUP, Que. — At least eleven elderly people have died since December at a seniors home east of Quebec City, most killed by a mysterious respiratory virus. Loved ones began to question staff at the St. Joseph home after a woman died on Dec. 22. Four other residents died in just six days, between Dec. 30 and Jan. 4. The son of one deceased says the death toll of 11 could be an underestimate. "There were others very recently," he said. "It's gone up to 19 deaths." Staff at the home imposed a quarantine and have banned lengthy visits. Four respiratory viruses, including flu-like influenza, have struck most of the residents and a health official says the outbreak has lasted longer than usual. "Currently, 85 of the 123 residents are affected," Brigitte Lavoie said. "Eleven died, but we can't say with certainty that this is directly related to the virus in all cases." Health officials speculate the culprit could be the human metapneumovirus, which can cause mild respiratory infections in healthy people. The symptoms can be more severe among the elderly and people with weak immune systems. "It's a big year for the virus," said Lavoie, adding some elderly people can catch the flu even if they've been vaccinated. "The older the person, the less effective the vaccine." Seniors homes in Quebec are required to report any outbreaks to infection control officials.