This is the first study to describe the PCFS Scale in a large sample of adult hospitalized and non-hospitalized subjects with confirmed and presumed COVID-19. The present results confirm the hypothesis that the PCFS Scale can be used to measure the impact of symptoms on the functional status of subjects after COVID-19, especially in slight to severe categories. We found gradual differences in the number and intensity of symptoms, reduction in HrQoL and impairment in work and usual activities between subjects with slight, moderate and severe functional limitation status (≥ grade 2) while no differences between subjects with no functional limitation (grade 0) and negligible functional limitation (grade 1) were found when comparing these outcomes, although a lack of power to detect small but meaningful differences among the latter groups might have affected these results.
The majority of the included subjects was not formally tested during the infection (78%), whereas a minority was hospitalized with confirmed diagnosis of COVID-19 (5%). These findings are expected, as early data showed that the proportion of critical cases of COVID-19 is 5%  and the number of undocumented infections have been estimated at 86% of all infections . When proposing the PCFS Scale, Klok and colleagues underlines the relevance of the assessment of functional status in the weeks and months following acute care of COVID-19 subjects . Our findings describe that, despite the inclusion of a middle aged sample of subjects without multiple comorbidities and with diagnosis of ‘mild’ COVID-19 (i.e. non-hospitalized during the infection), the majority of them (85%) report slight-to-moderate functional limitations during course of COVID-19, 3 months after the onset of symptoms, supporting the existence of a ‘post-COVID-19 syndrome’  which may be a target for deciding on referral of non-hospitalized subjects to expert (outpatient) clinics, facilitate selection of subjects who may benefit from rehabilitation programs as well as measuring the efficacy of such programs.
Of note, one of the strong points of the scale is that it intentionally does not specify why subjects are functionally impaired, i.e. if the impairment is caused by dyspnoea, fatigue, pain, anxiety or otherwise. This is crucial for the scale to be useful in a condition that may cause symptoms in many organ systems. However, it does not rule out the fact that the reported functional limitations are (partly) pre-existent. This is no issue when the scale is measured repeatedly or used as outcome measure in studies comparing two or more groups, especially in randomized controlled trials. The manual to the scale describes the possibility to assess pre-COVID-19 functional status, which would allow for determination of the causality of functional impairment in observational studies . Importantly, since the aim of the current study was to assess the associations between measures of symptoms intensity, HrQoL, impairment in work and usual activities due to health and the PCFS Scale, these considerations do not pose an issue for the correct interpretation of the presented data, nor does the fact that COVID-19 was not confirmed by objective testing in all subjects.
As most of the included subjects in our study were highly symptomatic and the PCFS Scale has shown to be associated with number and intensity of symptoms, the groups of subjects stratified as presenting no functional limitations (grade 0) and negligible functional limitations (grade 1) were relatively small (3% and 8% of total sample, respectively) likely due to a selection bias arising from the self-selection nature of the data. Also, the lack of statistically significant differences between these groups could be a direct consequence of this limited number of subjects. It is also possible that there is, in fact, not a clinically relevant difference in outcomes between these two grades of the PCFS Scale, future studies with inclusion of less symptomatic subjects are necessary to investigate this issue. After all, both a scale grade of 0 and 1 indicate no functional limitations. The clinical relevance between the 2 grades is possibly to differentiate full recovery (grade 0) from incomplete recovery and persistent symptoms (grade 1).
The groups of subjects stratified as presenting slight and moderate functional limitations (grade 2 and 3) present similar baseline characteristics, however with a gradual increase in number/intensity of symptoms, reduction in HrQoL and impairment in work and usual activities. Despite presenting significantly lower BMI, the relatively small group of subjects with severe functional limitations (grade 4) was found to be highly symptomatic and presenting worse HrQoL. This could be partially explained by the higher proportion of subjects with multimorbidity included in this group, since previous studies have found that multimorbidity is an independent factor associated with HrQoL [24, 25]. Furthermore, this group of subjects also presented a higher proportion of subjects with other factors that are associated with poorer functional status such as living alone and presenting a ‘symptom-based’ COVID-19 diagnosis.
The main limitation of the present study is the inclusion of a self-selected sample of subjects with confirmed and presumed COVID-19 who are still symptomatic after the infection, it is possible that the frequency and intensity of symptoms reported by non-hospitalized subjects is overestimated due to selection bias and due to the inclusion of a relatively high proportion of women, who usually report more symptoms than men . Moreover, if COVID-19 ICU survivors and other hospitalized subjects would have been included, the distribution of the PCFS could have shifted towards more symptoms and functional impairment. Future studies should investigate the distribution of the scale grades in less selected cohorts of COVID-19 subjects, as well as confirm the identified associations in other samples including a higher proportion of men, less symptomatic subjects and in sequential assessment of the scale during the course of disease (e.g. at the time of hospital discharge, more than 3 months after the onset of symptoms). In addition, we did not include any objective measure of physical functioning that could be related to functional status, however the strong association with the domain ‘usual activities’ of the EQ-5D-5L and WPAI and the ability to discriminate subjects with higher number/intensity of symptoms demonstrate the construct validity of the PCFS Scale.