With the virulence of SARS-CoV-2 had gradually weakened, China was fully released at the end of 2022. Due to the strict domestic controls, there had been no nationwide infection in China. After lifting the relevant containment measures, many infected people with the current circulating stain appeared. Because of its weak virulence, most infected people were mild and could recover within one to two weeks after infection, but for older infected people, they might develop severe or critical illness. In this study, we found the plasma antibody level of severe/critical group was significantly lower than that of mild group. And we wanted to find the factors of severe/critical patients. Clemens A. Schmitt et al.5 reported COVID-19 brought bigger influence in the elderly based on cellular senescence. Our study also found there existed association between plasma antibody levels and ages in SARS-CoV-2 infectors. It was a negative correlation between age and plasma antibody levels in our severe/critical group. That means in severe and critical patients, older age is more at risk. Besides, age was positive correlated with antibody levels in the mild group in this study. Combining the results of two groups, there may be an age interval where the correlation between age and antibody levels changes from positive to negative when the patient is older.
Strong epidemiological evidence exists that sex is an important biologic variable in immunity6. Some data demonstrate female immune system may generate stronger antibody responses7–10. Whether gender differences in the humoral immune response occur in COVID-19 is unanswered. In our study, it seemed that plasma antibody levels were not associated with gender. But it could be seen from Table 1 that the proportion of female patients was lower than male patients. More studies with larger sample sizes are needed to explore this association.
In addition to the factors above, we also found there existed a strong correlation between vaccination status and antibody levels in severe/critical patients. This suggests vaccination is meaningful for improving antibody levels and fighting with COVID-19.
There are several limitations to our study. These include the relatively small sample size, the lack of data on patients at younger ages, and the lack of clinical testing data. Besides, most patients in the mild group doesn’t have basic disease, expect two individuals, which may influence the analysis results of this part. However, all the data was collected based on the reality. And the current study design allows for a preliminary assessment of the factors in the severe and critical COVID-19 patients. At the time of conducting this study, there were no second infections among the included individuals. Further studies are needed to confirm our findings.