(CNN) — Flu activity increased over the past week, squelching hopes of an early peak and leading at least 12 states to shut down some schools.

There have been as many as 11.4 million flu illnesses, 5.4 million medical visits and 136,000 flu hospitalizations between October 1, 2018 and January 19, 2019, according to the weekly flu report released Friday by the US Centers for Disease Control and Prevention.

READ MORE: Watch The Derek Chauvin Trial Live

During the week that ended January 19, the CDC reported three additional children had died due to flu-related causes, bringing the total number to 22 for the season. For adults, flu deaths are estimated based on pneumonia and other illnesses related to flu. The proportion of deaths attributed to pneumonia and flu was 7.2%, which is slightly higher than the usual threshold of 7.1% for this time of year, the CDC said, based on data from the National Center for Health Statistics.

Flu activity is expected to remain elevated for weeks, with 18 states and New York City experiencing high activity last week, an increase over the previous week when only nine states reported the same.

CNN has unofficially counted school districts in at least 12 states that have closed for one day or longer as a result of flu outbreaks. They are: Alabama, Arkansas, Idaho, Illinois, Kansas, Michigan, North Carolina, Ohio, Oklahoma, Tennessee, Texas and Virginia. Private schools have also closed in additional states, including in California, Florida, New Jersey, New York, Louisiana and West Virginia.

READ MORE: City Of Detroit Suspends Use Of Johnson & Johnson COVID-19 Vaccine

The H1N1 virus strain is the most widely confirmed influenza A strain this season, however H3 viruses are dominating in the southeastern United States, the CDC reported Friday. Last year, the H3N2 strain, which is known to cause more severe illness, dominated.

Overall, the effectiveness of last year’s flu shot was estimated to be 40%, meaning getting vaccinated reduced an individual’s risk of seeking medical care by 40%.

Dr. William Schaffner, medical director for the National Foundation for Infectious Diseases, said that those who get the flu after receiving a vaccine are less likely to require hospitalization and they’re less likely to die.

“The vaccine is not perfect,” said Schaffner. “But give the vaccine credit for softening the blow.”

MORE NEWS: Man Of Michigan Teen Who Brought Explosive To School Charged By Feds

™ & © 2019 Cable News Network, Inc., a Time Warner Company. All rights reserved.