The masks are back, starting Friday.

City officials this week announced an indoor mask mandate for all Chicagoans ages 2 and up, regardless of vaccination status, for places like restaurants, bars and gyms. 

Also this week, U.S. health officials announced that a COVID-19 booster shot will be available for Americans starting in September, with nursing home residents, health care workers and emergency workers at the front of the line. 

“This will boost your immune response. It will increase your protection from COVID-19. And it’s the best way to protect ourselves from new variants that could arise,” said President Joe Biden. 

Chicago residents are currently facing a “very high risk” of contracting the virus, with the city’s seven-day rolling average of confirmed COVID-19 cases surpassing 400. 

Dr. Robert Murphy, director of the Institute for Global Health at Northwestern University’s Feinberg School of Medicine, said the booster shot will significantly increase people’s immune response to the virus. Booster shots will be available for people eight months after the second shot of their original Pfizer or Moderna vaccination.

“The protection from the vaccine starts to go down somewhere after six months and at eight months it’s clear the vaccine is not working as well,” said Murphy. 

That’s been shown through a rise in the rate of breakthrough cases, he said. However, those cases are not leading to an increase in hospitalizations. 

“I think what you’re seeing is the beginning of herd immunity,” Murphy said. “You can still get infected, but you don’t get severe disease, you don’t end up in the hospital and you don’t die. These vaccines work.”