key: cord-0048691-uv0m0vl3
authors: George, Aaron E.; Joshi, Maulik S.
title: You Need a Herd for Herd Immunity
date: 2020-07-23
journal: NEJM Catal Innov Care Deliv
DOI: 10.1056/cat.20.0348
sha: d36b8b11a20cc18d97457541293639bfc177a631
doc_id: 48691
cord_uid: uv0m0vl3

The limited prevalence of the novel coronavirus in non-major metropolitan areas presents its own unique challenges to health care leaders. In a low-density populated area of western Maryland, leaders of a community hospital focused aggressively on testing and, in partnership with the county health department, managed to suppress the Covid-19 curve before it got out of control.

would be years before the necessary "herd" was established to consider herd immunity. Given this path, the health and economic toll would simply be too grave to consider.

Anticipating a potential Covid-19 surge in our non-metropolitan community, we worked with the Washington County Health Department and all major stakeholders diligently to flatten the curve of hospitalization through public health interventions, access to testing, and the provision of care in all ways convenient to the patient. These measures achieved the goal to flatten the curve: as of July 11, 2020, Meritus health system's capacity was not exceeded, and we dramatically reduced the expected surge in hospitalizations. Predictive modeling done in March had indicated that there would be more than 200 Covid-19 hospitalizations by early April, but through aggressive testing and preventive measures, we were able to keep that number under 15 throughout. In considering the best way to provide easy access to Covid-19 testing, our health system, Meritus Health, focused on a strategy that has allowed us to process more than 18,000 Covid-19 tests between mid-March and July 10, 2020, and more than 1,200 Covid-19 antibody tests from mid-May to mid-June 2020.

Our cumulative positive Covid-19 antigen rate from testing (Roche 6800 with positive and negative percent agreement each of 100%) equals 4.2% and positive antibody testing of first responders and health care workers is 2.6%. These include health care workers, EMS, firefighters, and police. These numbers are substantial considering the proportion of the overall population they represent, and the resulting potential implications of prevalence is significantly lower than what has been published in the literature and lay press. For instance, national data from the CDC suggests a 9% positive antigen test rate within the general population, and within Maryland such rates have been shown to be between 11%-20%.5 Months of social distancing has mitigated spread, yet has also resulted in economic toil, public frustration, and rising mental health concerns. In our nonmetropolitan community, we are quite far from the baseline 60% prevalence necessary to consider herd immunity. The time and cost of bridging that gap while retaining social distancing and related curbs on business activity would be too immense to consider. We recognize that slowing or stopping the pandemic will be a multifactorial approach. There is no one answer. However, a vaccine will be the next -and perhaps only -major milestone in stopping the pandemic.

Meritus Health is a health system serving Washington County (population of 150,000 people) in western Maryland. Our service area also includes the eastern panhandle of West Virginia and proximate parts of southern Pennsylvania, resulting in care for approximately 225,000 people. We are the only health system in Washington County, though other health systems surround us. Meritus Health has approximately 3,000 employees, 500 medical staff, a nearly 300-bed, level III trauma medical center, a 100-provider medical group, and a home health agency. U.S. Census data shows the population density of Washington County is just 322 per square mile; by contrast, Baltimore's population density is 7,671 and New York City's density is 27,709 per square mile. Washington County is a 60-to 90-minute drive to both Baltimore and Washington, DC. Though not defined as rural, we, like most of the communities in America, can be defined as a nonmajor metropolitan area.

At Meritus Health, beginning in early April, we utilized seven different prediction models and amalgamated them into one set of assumptions to estimate our Covid-19 hospitalization demand curve. Variables included population, case-doubling rate, length of hospital stay, social distancing effectiveness, as well as regional and national trends for spread of disease. Early predictive modeling in Washington County demonstrated Covid-19 cases and hospital volume would peak in mid-April. However, with each successive model assessment generated over subsequent weeks, the maximum number of predicted cases decreased, and the projected peak extended into late June and early July -effectively demonstrating a flattening of the curve. We believe this occurred as a result of the implementation of multiple interventions including, social distancing, masking, selfisolating, quarantine, cancelling and deferring elective surgeries, closing non-essential businesses, and the enactment of stay-at-home orders by the governor of Maryland (Figure 1 ).

Initial predictions had suggested an early surge in patient volume alongside community spread of Covid-19. In that scenario, the total number of new cases would have decreased rapidly by the summer months, and herd immunity may have been achievable by spring of 2021. That said, current models demonstrate successful flattening and concurrent prolonging of our course. In concert with lower-than-anticipated positive antigen and antibody testing numbers, it is becoming increasingly apparent that an overwhelming outbreak in our community may not immediately materialize. This mirrors the experience in other rural communities across the country and has ramifications on system planning and community activity in the months ahead.

The Covid-19 preparation and testing strategy in Washington County began in early March, in partnership between Meritus Health and the Washington County Health Department. The testing strategy and execution was driven by Meritus Health, with the major priority to provide frictionless access. Meritus Health established three pop-up Covid-19 screening sites in strategic locations throughout the community and the county health department added a fourth site. Each site performed serologic antigen testing for Covid-19. To minimize barriers, screening sites were geographically located to allow walk-up and drive-through testing in areas of high population density and lower socioeconomic grouping. Testing costs at the three Meritus Health sites were covered by Meritus Health to eliminate financial impediment and help to understand prevalence of disease, particularly in vulnerable populations. Early on, screening for Covid-19 was offered primarily for symptomatic individuals; however, testing was expanded in late April to the community at large without restrictions. Beginning in mid-April, all inpatients received Covid-19 testing.

Covid-19 antibody testing began in mid-May and included all health care workers and frontline public safety members (EMS, firefighters, and police) who had cared for or immediately been in contact with known Covid-19 positive individuals. Serological testing for SARS-CoV-2 antibodies utilized a qualitative IgG Diasorin diagnostics test with manufacturer advised sensitivity of 99.9% at 14 or more days after positive PCR confirmation and diagnosis, specificity of 99.8% and with no cross-reactivity with common cold coronaviruses. This test was also provided at no cost to the individuals receiving the test.

Initial results in April indicated 10% to 15% of symptomatic patients tested positive for the Covid-19 antigen or, in other words, were presumed to have active infection. (Figure 2 ) As the testing strategy expanded to include any community member, regardless of symptom status, the percentage of positive results subsequently decreased to less than 10%, with a total of approximately 3% positive in the month of June. (Figure 3, Figure 4 ) While this result follows from more asymptomatic individuals seeking testing, it does offer perspective on global spread of disease in a community with approximately 15% of the total population tested.6 Looking Ahead-Locally and Nationally

In our non-major metropolitan area, we moved swiftly to provide testing early to best understand the spread of Covid-19. As our testing capacity expanded, we concomitantly increased the access to those who could be tested in the county. As of July 9, about 400 people are tested daily for Covid-19, with more than 18,000 tests performed in Washington County and surrounding areas since mid-March. We were fortunate to have the capability to conduct antibody testing in our lab and have collected results for more than 1,200 tests as of July 10. Our prevalence is lower than anticipated, at 4.2% positive Covid-19 antigen tests and 2.6% with antibodies present.

There are of course limitations in our data as they are from a convenience sample and perhaps skewed to symptomatic individuals. There are also the known inherent false positive and false negative rates that these tests carry, though such are consistent biases in all data. Fundamentally, we believe we are close to a population-level estimate based on the number of tests performed for our population size. Testing has been performed on about 10% of the service area population and has been aimed at a more generalized sample with a mix of geographic and symptomatic as well as asymptomatic individuals.

Meritus Health and the Washington County Health Department have partnered in a unified command structure in the management of the pandemic for the county. Co-leads were identified between Meritus Health and Washington County Department of Health in all critical functions of incident command -logistics, operations, and communication. A unified incident command means that we are "joined at the hip" in terms of frequent communication, which includes daily calls and a deep connection at the operational levels.

Early preparation, messaging, and social distancing interventions during this first phase of the pandemic have accomplished the goal of flattening the curve. For example, Washington County has 4.8 Covid-19 cases per 100,000 residents; our neighbor to the west, Allegany County, also is low at 3.1 per 100,000. But to our east, infection rates are notably higher: Frederick, 10.0, Montgomery, 15.0, Carroll, 7.2, and Baltimore, 10.6 per 100,000.7

As we increasingly recognize that the Covid-19 response will require several additional months or years, we have implemented a proactive monitoring system. In this, we identified 10 measures believed to require a heightened focus and responsive action. Our county is using this 10-measure dashboard to track the progress of Covid-19 during the initial reopening of the community and county, which began on June 3, 2020 ( Figure 5 ).

Given our shift from the mindset of immediate crisis response to a longer-term confrontation with Covid-19, we believe that each community should devise similar local monitoring parameters.

Meritus Health system is located in a catchment area with multiple states, serving patients from Maryland as well as Virginia, West Virginia, and Pennsylvania. Additionally, health care workers in Meritus Health as well as first responders in Washington County live and commute from these neighboring states. Therefore, as testing expanded, we did not place any geographic restrictions, and believed it a population health imperative to test broadly. This has created the potential for capacity issues for our own county and state residents, as well as added time for registration of many new patients at the point of testing. Intermittently, this has led to periodic bottlenecking and unanticipated wait times at our testing sites. Meanwhile, there has not been substantial communication or partnership with health departments in these neighboring states.

Observing concerning trends in surrounding states and national hotspots, both Meritus Health and the Washington County health department were able to convey early and targeted messaging on the importance of social distancing and appropriate utilization of masks. This coincided with rapid action at the state level and early orders from the governor's office and resulted in the community generally following stay-at-home orders. In particular, messaging to vulnerable and elderly patients was observed to be effective, with these groups avoiding exposure and limiting hospitalization.

It is becoming increasingly apparent that herd immunity may be a far-removed future reality. From a policy perspective, we cannot lose sight of this, and as our country re-opens in different phases and regions, we must continue to track and prepare for future peaks and surges until a vaccine is developed and deployed."

As the Covid-19 pandemic prolongs and continues, we are all left in an uncomfortable position of uncertain waiting. As of July 14, the U.S. death toll of more than 135,000 is appalling, and our hearts go to those we have lost and their loved ones. Nationally, a multitude of interventions in these past 90 days has had tremendous impact to limit these numbers from being even greater. Yet, despite these staggering figures, our non-metropolitan testing shows only an estimated 4.2% community prevalence thus far. It is becoming increasingly apparent that herd immunity may be a far-removed future reality. From a policy perspective, we cannot lose sight of this, and as our country re-opens in different phases and regions, we must continue to track and prepare for future peaks and surges until a vaccine is developed and deployed. This is our vital lever for health. Until that point, we are buying time.

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Percent of population tested for Covid-19 in Maryland counties

Confirmed cases per capita by Maryland jurisdiction, based on Maryland Department of Health data

The authors thank the contributions from the Meritus Health team (John Newby, MD; Melanie Heuston, DNP, RN, NEA-BC; Carrie Adams, PharmD; Michael Staley, PharmD; Mahesh Krishnamoorthy, MD; Shaheen Iqbal, MD; and Sara Abshari, MS) and the Washington County Department of Health for their unwavering leadership and commitment to our community.Disclosures: Both authors are employed by Meritus Health.