Since the lifting of epidemic measures in China, the COVID-19 infection rate has continued to rise. Within 3 weeks after the restrictions were lifted, a study showed that 70% of the population in Macau was infected with COVID-192. Many people are still plagued by a series of symptoms after contracting COVID-19, but there is no precise definition for these long-lasting symptoms. The WHO defines those with post-COVID-19 symptoms as individuals with possible or confirmed COVID-19 who still have symptoms 3 months after infection, symptoms that persist for at least 2 months, and symptoms that cannot be explained by other diseases3. The National Institute for Health and Care Excellence (NICE) defines this as symptoms that persist for more than 12 weeks after being infected with COVID-194. The definition adopted by the Centers for Disease Control and Prevention (CDC) is persistent symptoms or health problems 4 weeks after infection with COVID-195, and this study used the definition criteria of the CDC. We observed a wide range of these long-term symptoms involving major systems of the body, which indicates that in addition to direct infection, long-term symptoms may also be caused by endothelial damage, immune system disorders, and hypercoagulable states6. The quantification of viral RNA was at a persistently low level7, and this potential persistent infection may explain the occurrence of symptoms after COVID-19 infection.
In this population-based survey study, we assessed various potential risk factors for COVID-19 infection and found that age, area of residence, and number of vaccine doses were associated with infection risk. For the other factors investigated, including sex, blood type, smoking, and other chronic diseases, no statistically significant associations with infection risk were found. A large epidemiological study in China observed that the age distribution of COVID-19 cases was skewed toward an older population, with a median age of 45 years and a lower infection rate among children. Among people over 19 years of age, the likelihood of COVID-19 increased sharply8. A study in the UK also showed that children and adolescents had a very low risk of developing COVID-19, and the vast majority of confirmed infected children and adolescents had only mild or even no symptoms9, which is consistent with our findings. Compared with young people, some people who are older or have underlying diseases may reduce their risk of infection by reducing social activities and the number of times they enter and exit public places. A large proportion of the population is still unvaccinated10, which may be the reason for the reduced prevalence and infection risk of the unvaccinated population. The risk of infection in urban areas may be significantly higher than that in rural areas due to the dense population and frequent social contact. In our study, there was no significant difference in the susceptibility of people of either sex to COVID-19, but some studies have suggested that the severity and mortality of COVID-19 among men are higher than those among women11. At present, many studies have shown that the A blood type is associated with an increased risk of infection, while the O blood type is associated with a reduced risk12. Our study failed to reach this result, and some studies have found that the ABO blood type is not associated with susceptibility13. Although studies have shown that nicotine upregulates the expression of angiotensin-converting enzyme 2 (ACE2) and increases susceptibility to COVID-1914, we included smoking in the multivariate regression model analysis, and the results were not statistically significant. This study also showed that older age, male sex, and underlying diseases such as hypertension, coronary heart disease, and COPD did not increase the risk of COVID-19, and there was no association between chronic kidney disease and susceptibility, which is consistent with the findings in the study by de Lusignan et al.15. However, study findings have been inconsistent. This discrepancy may be related to the fact that the questionnaire we used affected the reliability of the results, or it may have been a false positive result caused by insufficient adjustment for potential confounding factors in other studies.
This study has some limitations. Since the scope of the survey focused on COVID-19 infections after the lifting of epidemic prevention and control measures in Anhui Province, there were certain limitations, and there was no comparison with the infection situation in other regions. In addition, the content of the questionnaire was written by one of the authors. The questionnaire questions included multiple-choice questions and fill-in-the-blank questions; thus, it was difficult to ensure the reliability and validity of the questionnaire. Due to the age structure of internet users, there may have been some imbalance in the age distribution of respondents, and the questionnaire format made it difficult to rule out the possibility of false positives or false negatives in the sample, and asymptomatic infections could not be identified. Due to the controversial diagnostic criteria for post-COVID-19 symptoms, there is currently no uniform biomarker or imaging test that can be used to confirm the diagnosis. Symptoms such as mild fatigue and decreased concentration are common in the general population and cannot be ruled out clinically. Psychological or other factors interfere, and the descriptions of symptoms after COVID-19 infection in the survey were all from the self-reports of the respondents. In short, this study evaluated the current situation, and the relationship between the research factors and the conclusions obtained is exploratory. Further observation and more in-depth research are needed to clarify this correlation.