Long COVID in Elderly Patients: An Epidemiologic Exploration Using a Medicare Cohort
Authors: Fung Kin Wah , Baye Fitsum , Baik Seo H , Zheng Zhaonian , McDonald Clement J 
Published: February 2023

Background

Incidence of long COVID in the elderly is difficult to estimate and can be under-reported. While long COVID is sometimes considered a novel disease, many viral or bacterial infections have been known to cause prolonged illnesses. We postulate that some influenza patients might develop residual symptoms that would satisfy the diagnostic criteria for long COVID, a condition we call “long Flu”. In this study, we estimate the incidence of long COVID and long Flu among Medicare patients using the World Health Organization (WHO) consensus definition. We compare the incidence, symptomatology, and healthcare utilization between long COVID and long Flu patients.

Methods and Findings

This is a cohort study of Medicare (the U.S. federal health insurance program) beneficiaries over 65. ICD-10-CM codes were used to capture COVID-19, influenza and residual symptoms. Long COVID was identified by a) the designated long COVID-19 code B94.8 (code-based definition), or b) any of 11 symptoms identified in the WHO definition (symptom-based definition), from one to 3 months post infection. A symptom would be excluded if it occurred in the year prior to infection. Long Flu was identified in influenza patients from the combined 2018 and 2019 Flu seasons by the same symptom-based definition for long COVID. Long COVID and long Flu were compared in four outcome measures: a) hospitalization (any cause), b) hospitalization (for long COVID symptom), c) emergency department (ED) visit (for long COVID symptom), and d) number of outpatient encounters (for long COVID symptom), adjusted for age, sex, race, region, Medicare-Medicaid dual eligibility status, prior-year hospitalization, and chronic comorbidities. Among 2,071,532 COVID-19 patients diagnosed between April 2020 and June 2021, symptom-based definition identified long COVID in 16.6% (246,154/1,479,183) and 29.2% (61,631/210,765) of outpatients and inpatients respectively. The designated code gave much lower estimates (outpatients 0.49% (7,213/1,479,183), inpatients 2.6% (5,521/210,765)). Among 933,877 influenza patients, 17.0% (138,951/817,336) of outpatients and 24.6% (18,824/76,390) of inpatients fit the long Flu definition. Long COVID patients had higher incidence of dyspnea, fatigue, palpitations, loss of taste/smell and neurocognitive symptoms compared to long Flu. Long COVID outpatients were more likely to have any-cause hospitalization (31.9% (74,854/234,688) vs. 26.8% (33,140/123,736), odds ratio 1.06 (95% CI 1.05-1.08, p<0.001)), and more outpatient visits than long Flu outpatients (mean 2.9(SD 3.4) vs. 2.5(SD 2.7) visits, incidence rate ratio 1.09 (95% CI 1.08-1.10, p<0.001)). There were less ED visits in long COVID patients, probably because of reduction in ED usage during the pandemic. The main limitation of our study is that the diagnosis of long COVID in is not independently verified.

Conclusions

Relying on specific long COVID diagnostic codes results in significant under-reporting. We observed that about 30% of hospitalized COVID-19 patients developed long COVID. In a similar proportion of patients, long COVID-like symptoms (long Flu) can be observed after influenza, but there are notable differences in symptomatology between long COVID and long Flu. The impact of long COVID on healthcare utilization is higher than long Flu.

Author summary

Why was this study done?

The quoted incidence of long COVID varies widely because of differences in definition and measurement method. Long COVID in the elderly is likely to be under-reported because they are less likely to respond to surveys, and symptoms may be confused with other chronic diseases. We describe a method of identifying long COVID in the elderly using a standard definition.

Lingering ill health after infections is not limited to COVID-19. We postulate that some patients may fit the diagnostic criteria of long COVID after a bout of influenza. We call this condition “long Flu”. Comparing and contrasting long COVID and long Flu may shed light on the understanding of long COVID, a disease still shrouded in mystery.

What did the researchers do and find?

We applied the World Health Organization consensus clinical definition to identify long COVID among 2 million Medicare patients who were diagnosed with COVID-19 between April 2020 and June 2021.

We applied the symptom-based long COVID definition to almost 900,000 influenza patients during the 2018 and 2019 Flu seasons to identify long Flu.

Long COVID occurred in 16.6% of outpatients and 29.2% of inpatients. The corresponding rates for long Flu were 17% and 24.6%. If one had relied solely on the designated diagnostic code to identify long COVID, the estimated rates of long COVID would be 0.5% and 2.6% among outpatients and inpatients, way below the reported rates in most studies.

Despite the similar overall incidence rates, long COVID patients suffered more often from difficulty in breathing, fatigue, palpitations, loss of taste/smell, memory problems, cognitive impairment and sleep disturbance than long Flu patients.

Long COVID patients were also more likely to be admitted to hospital and had more outpatient visits on average than long Flu patients.

What do these findings mean?

The use of designated long COVID diagnostic codes alone is likely to result in gross under-reporting.

A similar proportion of influenza patients suffer from a prolonged illness resembling long COVID, but there are notable differences in the incidence of individual symptoms between long COVID and long Flu.

Long COVID is associated with higher level of healthcare utilization than long Flu. This means that long COVID is likely to have a bigger impact on the individual’s health as well as on society as a whole.