Research Letter| Volume 22, ISSUE 2, P256-257, February 2021

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Development of Herd Immunity to COVID-19 in Louisiana Nursing Homes

Published:November 28, 2020DOI:
      The State of Louisiana Department of Public Health has been publishing the nursing home Coronavirus Disease 2019 (COVID-19) statistics of the 281 nursing homes in the state since April 18, 2020.
      Louisiana Department of Health
      COVID-19 Long-Term Care Facilities.
      , The following are the results of analysis of these data. A database was created containing the name of the facility, the initial census, the COVID prevalence, and the mortality for each of the 23 weeks of their reported data.
      The patterns of COVID prevalence in each nursing home were determined. Five patterns emerged: (1) homes that already had a COVID cluster before the data collection started; (2) homes not yet having a COVID cluster; (3) homes still in the throes of a COVID cluster; (4) homes having completed a cluster of COVID during the period of the data collection; and (5) homes with more than one cluster.
      Group 4 was specifically analyzed, as there were sufficient data available during the entire course of the infective cluster. The remaining 4 were not analyzed because of either no data or missing data.


      As the peak of the COVID cluster occurred at a different times in each nursing home, the time frame of the cluster was shifted so that its peak was set at zero time and the incidence for each week was accumulated. A logistic population model dx/dt = x ∗ (1-x) dt was applied to these data rather than a 3-compartment model such as the SIR (Susceptible, Infected, Recovered or Removed) model, as data for the resolution of the infection were unavailable. The progression of the epidemic is shown in Figure 1. The curve of the predicted model closely matches the actual; the difference is seen in the figure, χ2 = 0; P = 1.0. It can be concluded that this model is applicable to the COVID pandemic.
      Figure thumbnail gr1
      Fig. 1The progression of the COVID infection in Louisiana nursing homes with cluster peaks adjusted to day 0. The total infected was 60% of the total nursing home population. The curve closely approximates the theoretical logistic population model. The rate of growth of the infection was 14.7% per day.
      There were 120 nursing homes having resolution of the COVID cluster in group 4.
      The proportion of infected residents was 59.8% of the nursing home population. The case mortality rate was 19.7%. The 95% probability limits of the infection rate lay between 45% and 75%. The daily rate of infection was 14.7% per day. The most notable observation was that once the cluster was completed, there were only sporadic cases of new COVID in those nursing homes. The single exception in group 5 was that 3 nursing homes had a second cluster, but the sum of the 2 clusters was not greater than the homes with a single cluster. An R value cannot be determined in a 2-compartment model. The COVID infection rate by combining the nursing homes into a single population over the 23-week data set also closely tracks the epidemiologic model.

      Conclusions and Implications

      Nursing homes are in effect a natural experiment with regard to the COVID epidemic. Each of the nursing homes is isolated from the general population and the social distance of the residents is quite low. There is been a lot of talk of herd immunity in the media. In this nursing home population, the cluster burned out when approximately 60% of the residents were infected. There has not yet been a resurgence of the COVID cluster in any of these homes. Either herd immunity was achieved or approximately 40% of the population was resistant. As a result of deaths and normal patient turnover, at some point there will be a sufficient susceptible population and so recurrent clusters may result.
      If the nursing home experience can be applied to the pandemic here in the United States, then the pandemic probably will not abate until 60% of the entire population is infected. The nursing homes that have had COVID clusters are already at the 100% point. The nursing home population in Louisiana is now at the 90% point on the S curve. The prevalence of the US population is 3% at the knee of the curve; a rapid growth of new cases can be expected. The acceleration of new cases cannot be attributed to noncompliance of masks or social distancing, but due to the natural history of this infection. The current rate of increase is 1.4% per day. The pandemic has hardly gotten started.
      Data and analysis are available at [email protected] on request.

      Supplementary Data


        • Louisiana Department of Health
        COVID-19 Long-Term Care Facilities.
        (Available at:) (Accessed January 21, 2021)
      1. New York Times. COVID-19 data.
        (Available at:) (Accessed January 21, 2021)