The coronavirus outbreak tied to the annual motorcycle rally in Sturgis, S.D., ended up generating more than $12 billion in public health costs, according to a new discussion paper.

Why it matters: The analysis puts a point on just how bad these superspreader events can be — and the difficulty of preventing them solely with voluntary policies.

Background: The annual rally was held this year over 10 days in August, and included a Smash Mouth concert. The nearly 500,000 attendees came from all over the country, and social distancing and mask-wearing were mostly optional.

• The visitors then left the city — which has a population of about 7,000 — and returned home, often taking the virus with them.

By the numbers: The rally led to266,796 additional cases, or 19% of the new cases in the U.S. between Aug. 2 and Sept. 2., the paper found.

• The event led to a 35% increase in cases in South Dakota. In counties that are home to the highest number of rally attendees, cases rose by 10.7% compared to counties without any attendees.
• If each coronavirus case costs $46,000, that's an additional $12.2 billion added on to the pandemic's price tag.
• "This is enough to have paid each of the estimated 462,182 rally attendees $26,553.64 not to attend," the authors write.

The other side: "Overall, I think the 'Sturgis Effect' that the authors document is in large part just a Midwest surge that took place during this time period. There is likely still a small Sturgis Effect...but the results are likely biased upward," tweeted Devin Pope, a professor at the University of Chicago.

The big picture: Given the state of contact tracing in the U.S. (bad), we'll never know how many coronavirus cases were actually tied to the Sturgis rally.

• But it's a reminder that it takes collective action to contain the virus: As Sturgis revelers head back home, this South Dakota-centered outbreak has the potential to infect people who never went anywhere near Sturgis and thought they were doing everything right.