Why are the data on long COVID so confusing?

Hi folks! I haven't written about long covid in a while so I wanted to discuss why it seems like everywhere you look there seems to be a different estimate of the prevalence of (or how many people currently living with) long covid.

EVOLUTION

In February of 2020, the WHO had reported that the recovery from COVID-19 took between 2-6 weeks. Shortly thereafter anecdotal reports from patients started to emerge of persistent symptoms lasting longer. Support groups subsequently formed on social media and these patients labeled themselves “COVID long haulers”. It has since become a national initiative; In 2021 the National Institutes of Health began the $1.15 billion dollar RECOVER program to sponsor studies to learn more about the long-term effects of COVID-19.

VARIABLE PRESENTATION

Long COVID symptoms vary from person-to-person and often with effects throughout different body systems. Studies have cataloged over 50 symptoms of long COVID but the most common symptoms occurring in at least 15% of long COVID patients are fatigue, post-exertional malaise, dizziness and brain fog.

WHY IS THE DATA SO CONFUSING?

The reason long COVID is not fully elucidated is because there are actually many challenges involved with studying long COVID, and its prevalence. In addition to knowledge gaps that we have on this relatively new condition, there is a lot of heterogeneity between the studies in the published literature. For example, there are a variety of different study designs used, varying time frames for what constitutes “long”, differences in the actual clinical case definitions between studies (symptoms), underreporting of the condition in real-world data, a lack of definitive testing (need to rely on a spectrum of symptoms which can also meet the criteria for other conditions), studies in inpatient vs outpatient settings include very different sets of patients, and limited duration of follow-up time since the condition is still realtively new. Because of these differences between studies, the prevalence estimates for long COVID have also varied widely.

PREVALENCE ESTIMATES

The CDC estimates long COVID to be between 5 and 30% due to heterogeneity in the study designs and case definitions. In addition to published studies, there are also prevalence surveys. The National Center for Health Statistics (NCHS) household pulse survey asks adults if they have ever experienced long COVID (defined as having had COVID and had symptoms that lasted three months or longer). Results of this prevalence survey from June 2022 through July 2023 have been remarkably consistent at between 14-15%. The Kaiser Family Foundation also report similar results from their survey, between 11-19%.

A study that just came out this week found that even mild cases of COVID-19 can result in long term symptoms lasting up to two years post infection, and patients who had a severe case of covid were at increased risk of persistent health problems and death for two years post-infection.

WHAT IS NOT CONFUSING

What has been consistent across studies are some risk factors that increase the likelihood that someone will develop long COVID: more severe covid-19 illness, especially those who were hospitalized or needed ICU care, people who had underlying chronic health conditions prior to their COVID-19 infection, people who did not get a covid-19 vaccine, and those who developed multisystem inflammatory syndrome (MIS) after infection.