October 9, 2020

How many coronavirus cases constitute an 'outbreak'? It's a surprisingly tricky question.

Daily Briefing

    In the absence of federal standards, states have adopted significantly different data reporting standards, inconsistent definitions for "basic medical concepts," and variant approaches to disease control in response to the new coronavirus—creating a "patchwork" system that experts say has enabled the virus to spread, Chris Mooney, Sarah Kaplan, and Juliet Eilperin report for the Washington Post.

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    Background on metrics

    The spread of a disease is generally considered an outbreak when it occurs beyond the typical background level, according to Tara Smith, an epidemiologist at Kent State University—and that threshold gets lower for new or rare diseases.

    When looking at workplaces specifically, the Council of State and Territorial Epidemiologists (CSTE) defines an outbreak of the new coronavirus as "two or more laboratory-confirmed Covid-19 cases among workers at a facility with the onset of illness within a 14-day period, who are not epidemiologically linked, do not share a household, and are not a close contact of each other outside of the workplace during standard care investigation or contact tracing."

    According to Mooney and colleagues, CDC states that there is a "high risk" of Covid-19 transmission in schools if the positive test rate in the community is 5%. But beyond that, Mooney and colleagues report there is no federal standard to define an outbreak of the new coronavirus—and the result has been a "patchwork" response among states in which categorizations of and responses to the spread of the coronavirus vary widely by state.

    State standards and qualifications vary widely

    For example, Michigan qualifies a workplace outbreak as just two coronavirus infections within the same work location. Similarly, New York requires school buildings to shut down if two people in two separate classrooms become ill. And West Virginia recommends schools switch to remote learning if at least two outbreaks happen in two classrooms and mandates that all schools close if the coronavirus test positivity rate hits 5%.

    In contrast, however, Iowa will not announce an outbreak for many kinds of businesses until at least 10% of staff are actively infections. According to a Post investigation in September, the state cited that threshold to "justify withholding evidence that at least 117 people at the Agri Star Meat and Poultry plant … had been sickened with the coronavirus."

    Meanwhile, when it comes to schools, Iowa—which does not track or report coronavirus cases in schools—requires each to hold at least half of its classes in person unless 15% of tests for the entire community are positive—far exceeding CDC's guidance that 5% presents a "high risk" of transmission— and a minimum of 10% of students call in sick. According to a White House Coronavirus Task Force report obtained by the Post, Iowa is in the "red zone" for viral spread and has been advised to establish new metrics, such as those used by West Virginia, for reopening schools.

    Iowa is also among the few remaining states that have opted not to report Covid-19 case counts by Zip code, information that according to Mooney and colleagues is particularly useful in remote areas where counties may encompass large geographic regions.

    Overall, according to the Mooney and colleagues, a review of state policies determined that the identical set of statistics would spur significantly differing responses depending on what state they occurred in. For instance, Michigan would qualify a case rate of nine diagnoses per 100,000 residents as "very high"—resulting in the closure of restaurants and bars, a prohibition on in-person gatherings, and a mandate to switch to remote learning—while Oklahoma when faced with the same case rate would rank it as "low," triggering only recommendations that businesses and schools use increase hygiene measures, the Post reports.

    And even in states that have comparatively low bars for determining a workplace outbreak, the information isn't necessarily made public. For instance, in California, where three confirmed cases in a workplace qualifies as an outbreak, it took more than two months for the Merced County Department of Public Health to inspect Foster Farms, a large poultry processing plan that had started reporting coronaviruses cases in April—and because Foster Farms listed deaths as "resolved" cases, the county wasn't aware of the actual number of deaths until seven people had passed away.

    Overall, according to Mooney and colleagues, the plant—excepting a partial shutdown between Sept. 1 and 7—has remained open, with a total of 396 workers reporting confirmed cases and nine passing away. And during that time, staff at Foster Farms were not made aware of the coronavirus cases among their colleagues or the local health department's categorization of the spread as an outbreak, according to Elizabeth Strater, an organizer with United Farm Workers, a union that represents some of the company's employees.

    "Looking back, it's like a horror movie," Strater said, "and we just weren't aware of the other scenes."

    Stakeholders voice concerns

    Cyrus Shahpar, chief science officer for coronavirus response at the nonprofit Resolve to Save Lives, said he believes that states are first determining how they want to approach the coronavirus epidemic and then establishing standards that justify that planned course of action. "It ultimately means that the amount of risk that you have to live with depends on where you are in the United States," he said.

    Shahpar added that this patchwork approach is hindering efforts to curb the virus' transmission. "If I get things under control but my neighbor is using different criteria and allows more disease, then I'm still in trouble," he explained. "We're never going to get past this unless we get on the same page."

    Caitlin Rivers, an epidemiologist at Johns Hopkins University, echoed those concerns, noting, "Every jurisdiction choosing its own indicators makes it really hard for community members to understand what's happening with their outbreak and compare."

    Separately, David Michaels, who headed the Occupational Safety and Health Administration in former President Obama's administration, expressed concerns about those states, such as Iowa, that are relying on percentages rather than numbers to define outbreaks. That approach "displays a poor understanding of both public health and mathematics," he said.

    Others voiced frustration over the lack of CDC guidance on these matters, noting that it requires local health departments to develop individual responses to the epidemic, even though these departments typically have significantly less expertise than CDC.

    For instance, Jennifer Rombalski, the public health director for La Crosse, Wisconsin, said her team dedicated the first few months of the epidemic to establishing nursing home guidance, tools for local businesses, and coronavirus-reporting dashboards. "In a time like Covid-19," she said, "for us to be all working on creating guidance materials at the same time when it should just be so much more coordinated has been really frustrating."

    But citing Iowa specifically, Jeff Engel, a former executive director of CSTE, who now serves as its senior adviser for coronavirus response, said while the 10% threshold "seems like an unbelievable high bar," state epidemiologists frequently have to navigate political pressure, and Iowa in particular is highly dependent on its meat industry. "It's a constant tension between keeping the economy going and protecting the public health," he said.

    Amy McCoy, spokesperson for the Iowa Department of Health, added that states may need to adopt distinct approaches and definitions to ensure their response is tailored to the needs of each specific state. "The definition of an outbreak in workplaces is an example of how states can come together to share experiences and provide suggestions for how to approach these issues," she explained, "but each state still may need to adapt depending on public health needs, state laws, and population sizes and settings" (Mooney et al., Washington Post, 10/7).

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