In the present study based on data from a population-based cohort with young adults, we observed that post COVID-19 symptoms for two months or more were present among 16.5% of the participants with confirmed COVID-19. However, few of these had been seeking healthcare for post COVID-19 symptoms or received a diagnosis of post COVID-19. The most common symptom was an altered smell and taste, present in more than two thirds of those with post COVID-19, followed by dyspnea and fatigue. Compared to participants without post COVID-19, those with post COVID-19, especially post COVID-19 dyspnea and fatigue had more often been bedbound during the COVID-19 infection. We found no difference in the prevalence of post COVID-19 between the sexes and observed no major differences in lifestyle factors or chronic diseases linked to post COVID-19. However, having asthma and/or rhinitis before the pandemic were associated with post COVID-19 dyspnea, migraine with altered smell/taste, and lower self-rated health with fatigue.

The observed prevalence of post COVID-19 is lower than most previous studies7,17,22,23, although relatively similar to other population-based studies15,26,27. For example, data from the United States (US) Centers for Disease Control and Prevention estimated that 19% of those reported having COVID-19 in the past still had symptoms of long COVID at the time of the survey26. Another large cross-sectional study among US adults observed that 15% of the individuals with a positive COVID-19 test reported continued symptoms for more than 2 months after the acute illness15. However, a pooled analysis based on 54 studies and two medical record databases in the United States found a lower prevalence; 6.2% of the individuals with symptomatic SARS-CoV-2 infections developed at least one of three self-reported long COVID symptom clusters three months after symptomatic SARS-CoV-2 infection6. In our BAMSE cohort, we have previously investigated self-reported long-term symptoms of suspected COVID-19 during the first half year of the of the pandemic (February to August 2020)16. This study showed that 11% of the participants with suspected COVID-19 symptoms had long-term symptoms for at least 4 weeks. However, in the beginning of the pandemic, testing for COVID-19 was not yet available for the general public in Sweden, and therefore, we were not able to investigate confirmed disease at this point.

Previous studies have identified older age as a risk factor for post COVID-194,13, however28, few studies have focused specifically on young adults. In children, the prevalence of post COVID has been shown to be lower compared to adults, however a substantial number of children have been reported to be affected6,29. As in adults, a wide range of long-term symptoms have been reported in children, although with a higher prevalence of mental health problems28,29.

In contrast to most previous studies in adults9, we did not observe a significant over representation of females with post COVID-19, although there was a slightly higher prevalence among the females. The sex distribution of post COVID-19 has been found to differ depending on age with women more commonly affected in adulthood and up to approximately 60 years of age30, whereas men are over-represented in the older age groups4.

After a severe COVID-19 disease, breathing difficulties has been shown to be the most common post COVID-19 symptom21, while a disease treated in primary care was more often associated with tiredness/fatigue22,23,24. The most common post COVID-19 symptom in our cohort was altered taste and smell, present in 69% of the participants with post COVID-19 (11.3% of the participants with confirmed COVID-19). This is a higher prevalence than in most outpatient populations investigated4,22,23,31, although a prevalent symptom in the acute phase of the disease32,33. In some studies, olfactory dysfunction has been linked to mild COVID-1934, a finding in line with our results where dyspnea and fatigue, but not altered taste/smell, was linked to disease severity (being bedbound). In a recent large Norwegian cohort study including participants aged 35–65 years, altered smell and taste was found to be the most specific symptom for post COVID-19. The other symptoms could be grouped in two clusters, one with symptoms such as fatigue and poor memory, and one related to respiratory symptoms like dyspnea and cough, suggesting different aetiologies35. This study also found more severe disease to be associated to a higher risk for post COVID-19 symptoms in the two clusters mentioned35. Together with previous studies, our results support an association between disease severity and spectrum of post COVID-19 symptoms.

In the present study, several potential risk factors for post COVID-19 were investigated. We did not find early life factors including infections or respiratory symptoms to be associated with post COVID-19. Among the conditions reported at 24 years of age, there were no significant associations between lifestyle (tobacco use) or chronic conditions/diseases and post COVID-19, although both rhinitis and asthma were associated with post COVID-19 dyspnea. However, no association was found between lung function at 24 years of age and post COVID-19, neither spirometric nor measured with MBWO. COVID-19 in turn has not been shown to affect spirometric lung function in young adults with mild disease36. This, however, does not rule out other conditions in the lung, such as a decreased diffusion capacity, found to be a sequela often proportionate to disease severity COVID-1937,38.

Overweight was not related to post COVID-19 but was more common among those with post COVID-19 dyspnea (although the difference was not significant). These results are in line with the previously established link between overweight and an altered ventilation pattern as well as chronic inflammation and asthma39. In addition, we have previously reported an association between asthma and higher levels of concern of the own health and stress associated to COVID-1940, in all, suggesting the dyspnea in the post COVID-19 context to be a multifactorial condition.

Previous studies have observed that co-morbidities increase the risk for post COVID-1938. In the present study, we did not find doctors diagnosed depression, ADHD or ADD, to have any significant association to post COVID-19, although doctors diagnosed migraine was more common among the participants with altered taste or smell. However, the lower level of participants who reported to be “completely healthy” at 24 years of age may suggest that those with post COVID-19 symptoms have an increased susceptibility or other pre-existing diseases that were not assessed in this study.

A limitation with our study is that some of the confirmed COVID-19 cases were self-reported (based on positive PCR, antigen, or antibody test taken before vaccination). The antigen tests have been shown to be less accurate compared to PCR-tests, although with a relatively high positive predictive value (true positive)41. Also, not all participants may have been tested for COVID-19, especially those with mild disease, which may lead to an incorrect estimate of the true proportion of post COVID-19. A general limitation in investigation of post COVID-19 is the unspecific character of the symptoms, which may be attributed to conditions other than COVID-1942. As in all questionnaire-based studies, there is also a risk of recall bias when reporting previous symptoms. In addition, there was no question on whether the participants still had symptoms at the time of answering the questionnaire and therefore no possibility to investigate how many that eventually recovered from their post COVID. It is also shown that post COVID symptoms differs by viral variant15,43, and the results may therefore not be representative of current viral variants. Unfortunately, there was no available information about which viral variant the participants were infected with, and this could therefore not be investigated further. With regards to the time point of data collection, this study likely mostly included wild-type SARS-CoV-2 (for cases during 2020), the alpha-variant (dominated in Sweden in the first half of 2021) and the delta variant (dominated in Sweden in the second half of 2021)44.

The main strength of the study is the population-based design with a well-characterized study-population. Information on lifestyle factors and chronic diseases had been collected recently before the pandemic, as well as in early life. Another strength is the high response rate limiting the risk of selection bias, although this cannot be ruled out since participants with post COVID-19 symptoms may be more prone to participate in a COVID-19 directed follow-up. At the same time, it is also possible that the most severe cases to a lower extent answered the questionnaire, which may have led to an underestimation of severe post COVID.

In conclusion, post COVID-19 symptoms are common, also among young adults in the general population. We identified no major risk factors including lifestyle and chronic diseases present before the pandemic for being affected, suggesting other factors and mechanisms to be involved (including genetics and immune response)45. Although not life threatening, post COVID-19 could have a considerable impact on public health due to the high prevalence and long-term symptoms.

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