Kyle Summers holds a stop sign

Teacher Kyle Summers holds a stop sign as students cross 14th street after the first day of school at Washington Middle School Monday, Aug. 24. A recently released study from St. Louis University has found that mask mandates have significantly lowered the transmission rates of COVID-19 in St. Louis County and St. Louis compared to transmission rates in Franklin, Jefferson and St. Charles counties who did not have a mask mandate during the 12 weeks of the study. The Franklin County Commission's month-long mask mandate went into effect Friday, Nov. 20. 

A countywide mask mandate will likely cause a swift shift in the county’s positivity rate, if 80 percent of Franklin County residents and visitors to the county adhere to the mandate, according to a new study from St. Louis University. 

Enbal Shacham, the lead author of the study and a professor at St. Louis University’s College for Public Health and Social Justice, said residents likely can expect to see the rate of infection and daily positivity rates begin to decline within two or three weeks from Friday, when the mask mandate was implemented in the county.

The study — which was co-authored by Dr. Alex Garza, director of the St. Louis Metropolitan Pandemic Task Force, and is being peer reviewed for publication in medical journals — compared how mask mandates in St. Louis city and St. Louis County impacted the infection rate compared to counties in the St. Louis metro region without mask mandates. 

“Daily infection rate, decreased by 44 percent in St. Louis and St. Louis County within the first three weeks following the mandate,” Shacham said. 

Twelve weeks into the mandate, the average daily growth rate of coronavirus cases in the two urban counties was still 40 percent lower than in three metro counties. The mandates also reduced transmission rates among high-risk groups. 

Researchers first measured and compared the average daily increase in COVID-19 cases in St. Louis and St. Louis County with neighboring counties three weeks prior to mask mandates being implemented and three weeks following the implementation. 

On July 3, the date the mandates in St. Louis and St. Louis County were implemented, there were 210 COVID-19 cases in Franklin County and the overall positivity rate was 3.27 percent. Three weeks later, the number of COVID-19 cases in Franklin County was 390 and the overall positivity rate was 3.34 percent.

However, 12 weeks after the mask mandate was implemented, the county health department reported 1,732 people had contracted the virus as of Sept. 25. The overall positivity rate, which tracks the infection rate from March, was 5.99 percent. The 14-day positivity rate, however, was 11.23 percent.

As of Thursday, Franklin County’s overall positivity rate had climbed to 15.78 percent. The seven-day positivity rate was 28.2 percent, in the top 50 of highest positivity rates in Missouri. 

The St. Louis County seven-day positivity rate was 19.39 percent, and St. Louis city’s was 16.78 percent. 

During the same seven-day time frame, Jefferson County had a positivity rate of 28.86 percent and St. Charles County of 25.23 percent. 

“The simple act of wearing a mask is one of the cheapest and most effective tools we have against COVID-19,” said Shacham. “We now have the evidence that wearing a mask distinctly and dramatically reduces infection rates in a community.”  

Shacham said the study can serve as a model for the city of Washington. The study  found that from July 3 to September 25, Washington reported the highest positivity rate increases for the county.

“I would suggest to look at the number of places — the smaller stores, bars and restaurants — that are outside of Washington. (I think) what you will find is that the impact of a mask mandate would be that some people would go to those places, but most people are still going to be in Washington and will abide by mask mandate.”