In the analysis of 385, 370 survey respondents between June and December of 2022, we estimated that approximately 29% of individuals who had previously contracted COVID also experienced long COVID symptoms lasting more than three months. The estimation of long COVID prevalence in the US national level is lower than that reported by a Scottish study, where more than 42% of COVID survivors experienced persistent symptoms after six months [4]. It is important to highlight that the questionnaire utilized in the Scottish study included a total of 33 symptoms, all classified as long COVID conditions. In contrast, the House Pulse Survey which our study relied on includes only 12 symptoms. Therefore, the difference in how long COVID conditions are defined and the range of persistent symptoms between the Scottish study and ours could potentially contribute to the divergent results. Our results also demonstrate the prevalence of long COVID tend to be low in the oldest age group (65+), consistent with a recent study based on a population-representative sample of 3042 US adults [30]. The lower prevalence of long COVID among older people can be due to several factors such as survivor bias, lower rates of virus exposure, and higher rates of vaccination of old adults [31].
Our study suggests that people with long COVID are at increased risk of cognitive deficits in memory and concentration and the risk correlates with the severity of COVID symptoms during acute infection phase. More specifically, for asymptomatic patients, they do not have an increased risk compared to those who never got infected. These findings corroborate with the current existing studies investigating the impact of COVID on cognitive status [32]. Furthermore, we have examined the association between different sociodemographic factors and cognitive deficits in long COVID patients and found that females had higher risk of experiencing cognitive deficit than males. However, in a recent study of 72 mild-to-moderate COVID survivors, Henneghan et al. found no gender difference in the overall cognitive function measured by a neuropsychological test [33]. We note that their study only included a small sample size and focuses on mild-to-moderate cases, therefore, potential gender effects in cognitive outcomes may not be detected. In addition, our results suggest that the risk of cognitive deficit is the highest in the age group of 18–34 years compared to other age groups. A large cohort study based on patients in the US of Veterans Affairs healthcare database reached a similar conclusion, demonstrating that the risks of cognition and memory disorders decreased as age increased [19]. Given that young adulthood is still in the stage of cognitive development, even a mild disruption of underlying biological processes caused by COVID can have significant consequences on cognitive outcomes, possibly providing an explanation for the strong association between memory deficit and young age [34].
Among different race groups, this study show that the risk of cognitive deficit is the highest among people with multiple races and the lowest among Asians. Some prior studies on the association of long COVID with memory disorders did not include subjects with diverse races/ethnicity, potentially limiting the scope of their findings [35, 36]. There may be multiple factors for the racial disparity on cognitive outcome shown in this study. One possibility is that the biological effects of COVID-19 may differ due to genetic differences in various racial groups, consistent with a review on the genetic insight for COVID [37]. Furthermore, we suggest that individuals with the lowest education level (high school diploma or below) have a significantly higher risk of developing cognitive deficits compared to other subgroups. Individuals with lower levels of education often have lower economic status [38]. Their economic disadvantages limit their access to healthcare and making them less likely to receive timely and effective treatment for COVID-19, potentially increasing their risk for developing cognitive symptoms. Alternatively, the result about the impact of educational level could be explained by the passive cognitive reserve theory [39]. According to this theory, people with higher levels of education tend to have greater cognitive reserve, which can help maintain their cognitive performance level for longer periods of time and reduce the risk of cognitive decline and dementia [40, 41].
As our research represents the first major effort to investigate the prevalence of cognitive deficit among long COVID patients across different US states, it is difficult to find other similar researches to compare with. However, our results corroborate other findings in this study. Since race and educational levels are risk factors associated with cognitive outcome, the prevalence of cognitive deficits among US states can differ significantly if there are variations in race distribution and education levels among these states. Therefore, this finding not only corroborates that individuals’ sociodemographic characteristics contribute to their cognitive outcomes, but also suggests regional variability among different US states.
COVID-19 pandemic remains a constantly evolving situation. As more treatment for acute COVID becomes available and the number of individuals receiving vaccines and boosters increases, the epidemiology of long COVID may change over time [42]. Therefore, it is important to conduct further research to evaluate the potential impact of vaccination on reducing the risk of cognitive deficits among patients who have breakthrough infection, as well as to identify which sociodemographic groups may benefit the most from vaccination. This information can help guide public health planning and vaccine distribution efforts. Furthermore, while the scope of this study focuses on individuals aged 18 and above, future research can be conducted to study cognitive decline among children with long COVID as well. Cognitive deficits can have a significant impact on children’ academic and social functioning, potentially affecting their future opportunities. Understanding the extent of cognitive decline in this population group will help identify risk factors and guide the development of interventions, such as cognitive rehabilitation, to improve outcomes for affected children. Therefore, it is important to conduct research on the long term effects of COVID in all age groups to address the broader impact of the disease.
Limitations
This study has several limitations. First, the study relies on self-reported survey data, including the response to questions of long COVID symptoms and memory deficits. Compared to the objective physiological or cognitive tests, the self-reported responses may be inaccurate due to personal interpretation of the questions. As a result, the accuracy of the assessment on the association between the risk of cognitive deficit and long COVID across various sociodemgraphic groups might be affected. Second, the Household Pulse Survey has a relatively low response rate of about 6% and the survey data may be subject to sampling bias. For example, individuals who have more severe symptoms may be more likely to participate in this survey, leading to an overrepresentation of more severe cases in the data. Finally, although the study demonstrates strong associations between individuals’ long COVID status and their cognitive outcome, we cannot definitely establish a causal relationship between long COVID and cognitive deficit. Other confounding variables, such as individuals’ pre-existing health conditions, may also play a role in their cognitive outcome. Not considering pre-existing symptoms prior to COVID infection could result in an overestimation of the impact of long COVID. Therefore, it is important to supplement this study with other types of research, such as longitudinal studies, to better understand the impacts of long COVID on cognitive functions.