Antibody responses against SARS-CoV-2 in COVID-19 patients.

Dear Editor,

In December 2019, a novel human coronavirus, named severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2), which first emerged in Wuhan, China, has caused an international outbreak of coronavirus disease 2019 (COVID‐19). The COVID‐19 global pandemic has caused global public health crisis, as well as subsequent huge economic devastation. Unfortunately, to date, there is no specific antiviral treatment recommended for COVID‐19. Understanding of the host immune responses, especially adaptive immune responses to SARS‐CoV‐2 infection, are essential for formulating strategies for antiviral treatment, vaccination, and epidemiological control of COVID‐19. Currently, the antibody responses against SARS‐CoV‐2 remains largely unknown. We herein estimated the longevity of specific antibodies against SARS‐CoV‐2, and reported that antibodies waned over substantially in COVID‐19 patients after recovery.

In this retrospective study, a total of 42 COVID‐19 patients with data of serial serum immunoglobulin M (IgM) and IgG antibodies to SARS‐CoV‐2 were enrolled from Tongji Hospital of Huazhong University of Science and Technology between 26 January and 8 March 2020. All enrolled patients were confirmed to be infected with SARS‐CoV‐2 by real‐time reverse transcriptase polymerase chain reaction assays on nasal swab specimens. All cases were diagnosed and classified according to the guidelines for diagnosis and management of COVID‐19 (6th edition) released by National Health Commission of China. Clinical manifestations consist of four categories: mild, moderate, severe, and critical. Demographic data and clinical features are summarized in Table 1. The median age of the studied patients was 61 years (interquartile range, 52‐65 years), and 66.7% (28/42) were females. Among them, 14.3% (6/42) were in critical illness condition. The IgM antibody and IgG antibody against SARS‐CoV‐2 nucleoprotein and spike protein antigens in serum samples were measured using a commercially available magnetic chemiluminescent immunoassay (YHLO Biotech, Shenzhen, China) according to the manufacturer's instructions. The intra‐ and inter‐assay coefficients of variation for IgM are 5.4% and 8.7%, respectively; and for IgG are 4.60% and 7.3%, respectively. Serial serum samples were isolated for 14 to 60 days after the onset of the symptoms during hospitalization, and IgM and IgG antibodies to SARS‐CoV‐2 were measured within 24 hours after isolation. Seroconversion for IgG and IgM occurred in all patients. Antiviral IgM, as well as IgG antibodies were detectable after 14 days of onset. The titers diminished substantially and 38 of 42 patients had more than twofold declines within 14 days. IgM antibody was undetectable in 14 patients, and IgG titers were less than 100 AU/mL in 31 patients 60 days after the onset of the symptoms (Figure 1).

Table 1

Demographics and clinical characteristics of COVID‐19 patients

Patient ID	Gender	Age, y	Date of symptom onset	Symptoms	Coexisting disorder	Severity
YJ	Female	58	January 16 2020	Diarrhea, shortness of breath, sore throat	Hypothyroidism	Severe
SH	Male	65	January 25 2020	Fever, cough	Hypertension, diabetes	Severe
SM	Male	64	February 07 2020	Fever, shortness of breath	Hypertension, kidney stone	Severe
SX	Female	40	February 09 2020	Fever, cough, sore throat	No	Moderate
SY	Female	80	February 04 2020	Fever, poor appetite	Hypertension, coronary heart disease	Critical
LJ	Female	56	February 05 2020	Fever, cough	No	Severe
CM	Female	31	February 05 2020	Fever, cough	No	Moderate
XY	Male	67	February 08 2020	Cough	No	Severe
LF	Female	71	February 08 2020	Fatigue, cough	Hypertension	Moderate
YH	Female	48	January 17 2020	Fever	No	Critical
GC	male	72	January 13 2020	Fever, cough	Hypertension	Severe
JL	Female	57	February 08 2020	Cough	No	Moderate
BT	Male	53	January 26 2020	Fever, cough	No	Severe
MY	Female	51	February 04 2020	Fever, sore throat	No	Moderate
JH	Male	50	February 07 2020	Fever	No	Moderate
XX	Female	62	February 07 2020	Cough, sore throat	Cerebrovascular disease	Severe
HM	Female	58	January 27 2020	Fever	No	Moderate
YZ	Male	63	February 10 2020	Fever, cough	Prostatic hyperplasia	Severe
ZL	Male	59	January 25 2020	Fever, cough, sputum production	Hypertension, diabetes	Severe
GC	Female	65	February 11 2020	Fever, cough, shortness of breath	No	Moderate
XM	Female	68	January 29 2020	Fever, shortness of breath, headache	Hypertension	Severe
DX	Female	48	January 26 2020	Cough, sore throat	Hepatitis B infection	Moderate
QM	Female	55	February 08 2020	Cough	No	Severe
QX	Female	40	February 04 2020	Cough, diarrhea, chest distress	No	Severe
ZY	Female	80	February 15 2020	Chest tightness, shortness of breath	Coronary heart disease	Severe
XQ	Male	62	January 26 2020	Fever, chest distress, poor appetite	Hypertension	Critical
CY	Female	71	January 22 2020	Chest tightness, diarrhea	No	Severe
WY	Male	57	January 20 2020	Cough, sputum production, chest distress	Hypertension	Critical
XM	Male	74	January 31 2020	Fatigue, shortness of breath	Diabetes	Critical
SF	Female	52	February 01 2020	Fever, poor appetite	Hypertension	Critical
XH	Male	55	February 04 2020	Fever	Diabetes	Moderate
YM	Female	70	February 04 2020	Fever, diarrhea	Diabetes	Severe
XW	Female	61	January 27 2020	Fever, diarrhea	No	Moderate
HZ	Female	67	January 23 2020	Cough	Hypertension	Moderate
ZJ	Female	64	February 04 2020	Fever, cough	No	Moderate
QW	Male	74	January 30 2020	Fever, diarrhea	Coronary heart disease	Severe
MM	Female	28	January 31 2020	Fever	No	Moderate
TT	Female	35	January 19 2020	Fever, cough, fatigue	No	Moderate
MP	Female	57	February 08 2020	Fever, cough	Hypertension	Moderate
JR	Female	62	February 17 2020	Fever, cough	Coronary heart disease	Severe
ME	Female	61	January 29 2020	Cough, fatigue	No	Moderate
CR	Male	62	January 29 2020	Fever, cough	Diabetes	Moderate

Temporal changes in IgM and IgG antibodies against SARS‐CoV‐2 in COVID‐19 patients. A, Dynamic changes in virus‐specific IgM and IgG antibodies levels. The IgM and IgG antibodies were considered positive when their titers were greater than 10 AU/mL. Red dashed line indicates scale 10. B, Relative change in the IgM and IgG antibodies titers. Titers were normalized to the highest titer of each patient. COVID‐19, coronavirus disease 2019; IgM, immunoglobulin M; SARS‐CoV‐2, severe acute respiratory syndrome coronavirus 2

Humoral immune response provides the host with a first line of defense against reinfection, and the strength and persistence of this immune response correlates with protection. Previous studies have shown that circulating antibodies against SARS‐CoV peaked at 4 months after disease onset, and lasted for at least 2 years. Antibodies against Middle East respiratory syndrome coronavirus, including neutralizing antibodies, lasted for at least 34 months after the outbreak. To date, there are few studies in dynamic characterizing of the specific antibodies to SARS‐CoV‐2. Long et al showed that the median day of seroconversion for both IgG and IgM was 13 days post symptom onset, and both IgG and IgM titers plateaued within 6 days after seroconversion. Similarly, Zhao et al showed that the average antibody levels increased in a week, after the onset of the symptoms, and continuously elevated over the next 2 weeks. Neutralizing antibody provides important specific immune defense against viral infections in patients. Ni et al demonstrated that most recently discharged patients had strong humoral immunity to SARS‐CoV‐2, but one follow‐up patient was negative. Furthermore, both IgG levels and neutralizing antibodies started to decrease within 2 to 3 months after infection, and 40% of asymptomatic individuals, as well as 12.9% symptomatic individuals became seronegative in the early convalescent phase. One mathematical model also suggests a short duration of immunity after SARS‐CoV‐2 infection. We analyzed the dynamics of antibodies, and found that SARS‐CoV‐2 antibodies substantially decreased in about 60 days after the onset of the symptoms. Detecting neutralizing antibodies was not part of this retrospective study, and therefore the neutralizing activities of the detected IgG antibodies are unknown. Taken together, these findings suggest that the specific antibodies against SARS‐CoV‐2 might be very short‐lived in convalescent COVID‐19 patients. Additional examination of serial convalescent serum samples from COVID‐19 patients should be done to determine the extent and duration of antibody‐mediated immunity.

Cellular immune responses also participate in immune‐mediated protection of viral infection. Effective clearance of virus may need collaborative humoral and cellular immunity. The newly discharged patients had developed SARS‐CoV‐2‐specific T cells, which significantly correlates with the neutralizing antibody titers. Previous studies have shown that memory B cells waned over time with several years in recovered SARS patients. In contrast, specific T cell anamnestic responses have been shown to provide long‐term protection, even up to 11 years postinfection. Potential anamnestic B cell and T cell responses existing in recovered COVID‐19 patients remain unknown. Thus, further analysis of protective immunity to SARS‐CoV‐2 in a large cohort of convalescent COVID‐19 patients are urgently need to determine whether recovered patients present a protective response against reinfection and would therefore benefit from vaccination.

CONFLICT OF INTERESTS

The authors declare that there are no conflict of interests.

AUTHOR CONTRIBUTIONS

AD, YL, JP, and DX searched the literature, collected, analyzed, and interpreted the data. AD and DX drafted the manuscript. AD, HY, and DX conceived the study, designed the research, and revised the paper. All authors have read and approved the final manuscript.