Dynamic changes of serum SARS-CoV-2 antibody levels in COVID-19 patients.

To The Editor,

Coronavirus disease 2019 (COVID-19) caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) was first reported in December 2019 and spread rapidly and globally, leading to a worldwide pandemic currently. As of April 4, 2022, more than 489 million confirmed COVID-19 cases and 6 million deaths were reported to WHO around the world. Evaluating the durability of the humoral immune response to SARS-CoV-2 is essential to understand the susceptibility of COVID-19 patients to the same virus after recovered from the infection. Recently, it has been reported in neutralization assays using SARS-CoV-2 virus that neutralization antibodies titer is correlated with protection against reinfection. Serum immunoglobulin M (IgM) and immunoglobulin G (IgG) against SARS-CoV-2 were detectable within the first several weeks after symptom onset. To date, there are few data on long-term of antibodies response to SARS-CoV-2 after initial infection. Here, the aim of this study is to investigate the duration or stability of viral-specific humoral responses in COVID-19 recovered patients.

This was a retrospective study involving 61 hospitalized patients with laboratory-confirmed COVID-19 in the Renmin Hospital of Wuhan University, Wuhan, China from January 17, 2020 to March 7, 2020. Patients received follow-up visits form February 25, 2020 to January 29, 2021. Each patient had at least two antibody tests, with a total of 173 serum samples collected. The time points were defined as the days after symptom onset in which samples were collected. From the first to fourth time point, it was days 1–30, 31–60, 61–120, and 296–368, respectively. The serum antibodies against SARS-CoV-2 (IgM, IgG and neutralization antibodies) were detected by a chemiluminescent immunoassay (YHLO Biotech, Shenzhen, China). The samples were processed and analyzed according to the manufacturer's instructions. Values ≥ 10 AU/mL are considered positive. This study was approved by the Ethics Committee of Renmin Hospital of Wuhan University (Approve No. WDRY2020-K073). Continuous variables were present as median and interquartile range (IQR) and the Kruskall-Wallis test was applied. A p-value less than 0.05 was statistically significant. All statistical analyses and scientific graphics were made by using SPSS 20.0.

From 17 January 2020 to 7 March 2020, a total of 61 COVID-19 patients were recruited in our study. The median age was 34 years (IQR, 30–47). 72.13% (44/61) were women and 27.87% (17/61) men. 29, 53, 30 and 61 specimens were collected 1–30 days, 31–60 days, 61–120 days, and 296–368 days after the onset of symptoms, respectively. We illustrate the overall profile of serum IgM, IgG, and neutralization antibodies against SARS-CoV-2 from day 1 to 368 after illness onset in Table 1 and Fig 1 .

Table 1

Levels of serum antibodies at different times.

	1–30 (d)	31–60 (d)	61–120 (d)	296–368 (d)
IgM antibodies	22.79 (6.45, 150.97)	17.61 (3.65, 57.05)	31.58 (11.38, 125.22)	2.66 (1.16, 8.22)
IgG antibodies	86.67 (46.88, 208.04)	123.67 (56.22, 191.01)	158.94 (115.69, 232.16)	36.32 (14.68, 78.68)
Neutralization antibodies	35.32 (12.16, 160.60)	54.43 (23.98, 145.74)	80.13 (56.98, 202.36)	14.30 (7.77, 27.9)

Fig. 1

Dynamic changes in antibodies against SARS-CoV-2. (A–C) Dynamic changes in IgM antibody (A), IgG antibody (B) and neutralization antibodies (C) in representative patients over the monitoring period. P-values were calculated by Kruskal Wallis test. P-values < 0.05 were considered significant with *, **, and *** indicate p < 0.05, <0.01, and <0.001, respectively.

The median serum IgM level was 22.79 (IQR, 6.45, 150.97) AU/mL in 1–31 days, reduced to 17.61 (IQR, 3.65, 57.05) AU/mL in 31–60 days, increased to 31.58 (IQR, 11.38,125.22) AU/mL in 61–120 days, but the difference was not statistically significant. The IgM level in days 296 to 368 reduced to 2.66 (IQR, 1.16, 8.22) AU/mL, which decreased significantly compared with other time periods (Fig. 1). The positive rates of IgM were 65.52%, 58.49%, 80.00% and 22.95%, respectively (Fig. 2 ).

Fig. 2

Serum antibody positive rates at different times. The positive rates of IgM, IgG and neutralization antibodies reached the peak in 61–120 days and decreased significantly in 297–368 days.

The median serum IgG level was 86.67 (IQR, 46.88, 208.04) AU/mL in 1–31 days, increased to 123.67 (IQR, 56.22, 191.01) AU/mL in 31–60 days and 158.94 (IQR, 115.69, 232.16) AU/mL in 61–120 days, but the difference was not statistically significant. The IgG level in days 296 to 368 reduced to 36.32 (IQR, 14.68,78.68) AU/mL, which decreased significantly compared with other time periods (Fig. 1). The positive rates of IgG were 96.55%, 100.00%, 100.00% and 81.97%, respectively (Fig 2).

The median serum neutralization antibodies level was 35.32 (IQR, 12.16, 160.60) AU/mL in 1–31 days, increased to 54.43 (IQR, 23.98, 145.74) AU/mL in 31–60 days and 80.13 (IQR, 56.98, 202.36) AU/mL in 61–120 days, but the difference was not statistically significant. The neutralization antibodies level in days 296 to 368 markedly reduced to 14.30 (IQR, 7.77, 27.9) AU/mL, which decreased significantly compared with other time periods (Fig. 1). The positive rates of neutralization antibodies were 79.31%, 92.45%, 100.00% and 67.21%, respectively (Fig. 2).

Some studies performed at the beginning of the COVID-19 pandemic showed that the amount of anti-SARS-COV-2 IgM/IgG antibodies decreased in several months post onset of symptoms. , However, Gong X et al. found that anti-SARS-CoV-2 IgG can last for at least 9 months in patients with a history of natural infection. Kaygusuz S et al. also showed that both IgG and neutralization antibodies levels continued unabated after 9 months of follow-up. Yousefi Z et al. found that the level of anti-SARS-CoV-2 IgG antibody was detectable at their highest level for 3 months, and a certain amount of anti-SARS-COV-2 IgG antibody could be detected in the serum of recovered patients up to 15 months. The humoral immunity persisted for up to 18 months in patients with mild COVID-19. The maximum duration of neutralizing antibodies and IgG antibodies could be long-lasting based on Linear Mixed Models, especially IgG.

In our study, SARS-CoV-2 specific IgM levels decreased substantially to background levels in 297–368 days among 77.05% of COVID-19 patients. 100% of the patients were seropositive for IgG and neutralization antibodies in 61–120 days following symptom onset and this rate dropped to 81.97% and 67.21% in 297–368 days after onset. These findings suggest that IgG and neutralization antibodies against SARS-CoV-2 can last for nearly 1 year. Surprisingly, the titers of IgM, IgG and neutralization antibodies in 61–120 days were increased compared to 31–60 days. It could be due to subsequent exposure of the studied COVID-19 subjects. These data provide important insights into both the clinical application of serology in managing SARS-CoV-2 infection and the limited longevity of antibody responses, which may have impact on immunity after SARS-CoV-2 infection.

Our study has several limitations such as small sample size, retrospective study, and different sample collection time. The relationship between clinical features and antibody dynamics, such as disease severity, was not discussed.

According to the present study's results, despite the decrease in the amount of IgG and neutralization antibodies in 297–368 days, a certain amount of antibody could still be detected in most recovered COVID-19 patients.