Flu activity began increasing in early September in parts of the West, an unusually early start that has prompted infectious-disease experts to recommend considering a flu shot sooner than usual. Flu seasons typically begin in October or November and often start in the South before spreading elsewhere. This year, the share of flu tests coming back positive has risen steadily in Western states, including California, Washington, Hawaii and Alaska. Flu-related emergency department visits have also increased for weeks, according to Caitlin Rivers, an epidemiologist at the Johns Hopkins Bloomberg School of Public Health. Rivers said the uptick resembles levels more commonly seen in November or December, although flu activity remains low across much of the country. She said the early rise and its western starting point are both unusual. The reason for the timing is not clear. One possible factor is a newly evolved variant with a mutation that may help it evade existing immunity, particularly among teenagers. Alex Greninger, a virologist at the University of Washington, said his team was seeing flu levels more typical of Christmastime and that the variant appeared to be common there. The early spread could leave people less time than usual to get vaccinated before flu circulates more widely. Virologist Scott Hensley of the University of Pennsylvania said vaccination remains crucial and that people may want to get their shots early this year. The Centers for Disease Control and Prevention has not promoted flu vaccination as prominently as it typically does. A CDC spokesperson said the agency is developing a communications strategy on vaccination and other steps people can take during respiratory-virus season. The agency declined to provide an official for an interview. Demetre Daskalakis, a former CDC immunization center director who resigned last year, criticized the agency’s limited public messaging. He noted that flu can cause deaths among children as well as tens of thousands of hospitalizations and deaths overall. The early trend is a reminder that seasonal illness can begin before winter—and that local residents should watch guidance from their health departments and health care providers as flu activity changes.