Mapping of Carver County outbreak unmasks how COVID spreads

Hannah Friedlander didn't like what the genomic sequencing was telling her. Two children from one Carver County school not only had COVID-19, but their infections in late January came from the same, more infectious viral variant.

Then came a cluster in a local hockey team, followed by an outbreak at an area child care facility. All were unusual for their high "attack rates," meaning more people exposed to the virus ended up infected.

And, as it turned out, all were linked.

"When we get a report in a team, it's not one report or two reports," said Friedlander, a senior epidemiologist for the Minnesota Department of Health's COVID-19 response. "It's half the team is down."

The resulting investigation of the Carver County outbreak produced one of the most detailed maps of COVID-19 transmission in the yearlong history of the pandemic — a chart that looks like a fireworks grand finale with infections producing cascading clusters of more infections.

"This is some of the most amazing epidemiology I've ever seen," said Michael Osterholm of the University of Minnesota's Center for Infectious Disease Research and Policy.

An outbreak of at least 12 infections at one K-8 school was tied by one person to a sports team with five infections, which were linked to a local high school with more than 20 infections among students and teams. One student from that school had ties via a recreation center to two other people who tested positive.

At its basic level, the mapping provides a historical retelling of one of hundreds of community outbreaks in Minnesota, which has reported 6,777 COVID-19 deaths and 504,273 known infections with the coronavirus that causes the disease. The totals include six deaths and 1,400 infections reported Saturday.

State leaders said some links in the cluster show how people carried COVID-19 from one group to another, while others might be coincidences. Regardless, the mapping provides important understanding of how new viral variants such as B.1.1.7 could upset progress in Minnesota, where nearly 25% of the population and 78% of senior citizens have received vaccine.

"There does seem to be this high attack rate associated with this variant, and so it has provided us with some insight as we look at other parts of the state," said Dr. Ruth Lynfield, state epidemiologist.

Genomic sequencing of patient samples found 317 infections in Minnesota that involved B.1.1.7, including 33 of 140 cases in the Carver cluster, with more results pending.

Minnesota is known for this level of epidemiology in other areas, especially with its Team Diarrhea group unlocking the foodborne sources of national salmonella outbreaks in recent years. But the intensity and volume of COVID-19 cases over the past year prevented similar detailed investigations of the pandemic.

Accurate mapping requires genomic sequencing, which provides the genetic lineage of viral infections and reveals whether people were infected by the same sources.

Minnesota's public health lab and its partners have tripled sequencing activities in response to the variants — gaining more random COVID-19 samples but also targeted samples from areas that appear ripe for viral spread.

Mapping also requires detailed reporting and contact tracing interviews, which have seen declining success over the past year as a wearied public has become less likely to participate and to provide information that could result in quarantines or restrictions.

In the teeth of Minnesota's first COVID-19 wave last May, 27% of infections in communities could not be traced back to identifiable sources. Now, 44% are declared unknown.

State leaders feared resistance in Carver County — no parent or player wants to report a mild COVID-19 case that puts an entire team in lockdown — but found a community of school and sports leaders willing to pitch in.

Schools, sports, children and the local community aren't to blame, said Cynthia Kenyon, a state epidemiologist supervisor. "That is not it at all. This virus is so transmissible that if it gets into the right social network, it just goes."

The first piece of the cluster was spotted in a private K-8 school, which served as an incubator of sorts because its students live in different towns and play on different club teams — each representing a new source of viral spread.

The challenge for epidemiologists was to catch up, given that they were learning in late February about infections that had occurred weeks earlier and had resulted in community spread of the virus.

By identifying cases and contacts in a cluster, "we can try to cut if off at the knees or maybe get ahead of it," said Susan Klammer, who leads the state's COVID-19 tracing in K-12 and child-care facilities.

The cluster ensnared four private schools, nine public schools, one child care center, 18 hockey teams, four basketball teams, three lacrosse teams, one soccer team and three fitness or recreation centers. Transmission occurred across county lines, with a B.1.1.7-driven outbreak among most girls in one Shakopee hockey team having a link back to the Carver cluster.

Statewide, 15 hospitalizations and two deaths have been found in people with B.1.1.7 forms of COVID-19.

An unlinked outbreak of 10 infections was found at a Perkins restaurant in Carver County, and nine cases involved a local American Legion post.

State epidemiologists gathered enough evidence to recommend one of the first new restrictions in Minnesota's COVID-19 response in weeks — when declining infection rates statewide had Gov. Tim Walz pulling back other restrictions. On March 5, they recommended, but did not require, a two-week pause on sports in Carver County.

"Some sports have more contact than other sports," said Friedlander, but some transmissions likely occurred in social gatherings rather than in games or practices.

The warning had an immediate impact. Players on the Minneapolis girls varsity hockey team were ready to take the ice for a game in the Chanhassen arena March 6 when they learned of the concern and voted on whether to play. Coach Sarma Ozmen had the girls close their eyes before raising their hands. Only one voted to play.

"We've had some teams that are not masked up properly," she said, "and it makes them nervous."

Four districts in Carver County opted to continue varsity athletics during the pause but to end the seasons of their JV and lower teams.

The hard part about pandemic response is that officials will never know what they prevented through their actions, but they do see some of the costs.

The WEST Express Swim Team had no links to the cluster but lost access to school pools and had to cancel a state meet for which its young athletes had long been training.

Head coach Heidi Miler said it was unfair, but she also has adult nurses on her team and decided to stand with health officials to publicly promote COVID-19 testing for athletes.

"I wasn't happy about getting shut down as we haven't had any COVID cases at the pool," she said, "but I also understand all of the mitigation efforts to stop the spread."

The detailed mapping of the Carver cases isn't repeatable for every cluster, but epidemiologists believe this event will better prepare the state.

"We don't have to watch it happen in each community," said Kris Ehresmann, state infectious disease director. "We can see the pattern and take action and intervene."

Jeremy Olson • 612-673-7744