Long-Term Kinetics of SARS-CoV-2 Antibodies and Impact of Inactivated Vaccine on SARS-CoV-2 Antibodies Based on a COVID-19 Patients Cohort

Abstract

Background: Understanding the long-term kinetic characteristics of SARS-CoV-2 antibodies and the impact of inactivated vaccines on SARS-CoV-2 antibodies in convalescent patients can provide information for developing and improving vaccination strategies in such populations.

Methods: In this cohort, 402 convalescent patients who tested positive for SARS-CoV-2 by RT-PCR from 1 January to 22 June 2020 in Jiangsu, China, were enrolled. The epidemiological data included demographics, symptom onset, and vaccination history. Blood samples were collected and tested for antibody levels of specific IgG, IgM, RBD-IgG, S-IgG, and neutralizing antibodies using a the commercial magnetic chemiluminescence enzyme immunoassay.

Results: The median follow-up time after symptom onset was 15.6 months (IQR, 14.6 to 15.8). Of the 402 convalescent patients, 44 (13.84%) received an inactivated vaccine against COVID-19. A total of 255 (80.19%) patients were IgG-positive and 65 (20.44%) were IgM-positive. The neutralizing antibody was 83.02%. Compared with non-vaccinated individuals, the IgG antibody levels in vaccinated people were higher (P=0.007). Similarly, antibody levels for RBD-IgG, S-IgG, and neutralizing antibodies were all highly increased in vaccinated individuals (P<0.05). IgG levels were significantly higher after vaccination than before vaccination in the same population. IgG levels in those who received 'single dose and ≥14d' were similar to those with two doses (P>0.05). Similar conclusions were drawn for RBD-IgG and the neutralizing antibody.

Conclusion: 15.6 months after symptom onset, the majority of participants remained positive for serum-specific IgG, RBD-IgG, S-IgG, and neutralizing antibodies. For convalescent patients, a single dose of inactivated vaccine against COVID-19 can further boost antibody titres.

Keywords: SARS-CoV-2; antibody responses; long-term kinetics; natural infection; vaccination.

Figure 1

Timeline of the two rounds of follow-up. A total of 402 COVID-19-cured patients (confirmed from January to June 2020) participated in the follow-up visits. Of these, 284 underwent the first round of follow-up (blood draw period from 26 August to 28 October 2020) and 318 participated in the second round of follow-up (blood draw period from 8 December 2020 to 21 June 2021).

Figure 2

Dynamics of the IgG positive rate and antibody levels during the two follow-up rounds. The length of follow-up ranged from 5.4 months to 17.4 months. At two-month intervals, the horizontal axis has been divided into seven equal parts, 5~6 months, 7~8 months, 9~10 months, 11~12 months, 13~14 months, 15~16 months and 17~18 months. (A) Trends in IgG positive rate over months after symptom onset in overall, unvaccinated and vaccinated convalescent patients. (B) Trends in IgG antibody levels over months after symptom onset in overall, unvaccinated and vaccinated convalescent patients. (C) Trends in neutralising antibody levels over months after symptom onset in overall, unvaccinated and vaccinated convalescent patients. P values were determined applying a two-tailed Mann-Whitney U test. P<0.05 was considered to be statistically significant.

Figure 3

Evaluation of the effectiveness of vaccination on convalescent patients. The participants were divided into 4 parts according to the dose and the inoculation days between the sampling and the last vaccination: “1 dose and <14d”, “1 dose and ≥14d”, “2 doses and <14d” and “2 dose and ≥14d”. (A) Effect of different doses and inoculation days on IgG antibody levels in two rounds of follow-up. (B) Effect of different doses and inoculation days on RBD-IgG antibody levels. (C) Effect of different doses and inoculation days on neutralising antibody levels. P values were determined applying a two-tailed Mann-Whitney U test. P < 0.05 was considered to be statistically significant (ns: no significance; ***p < 0.001; ****p < 0.0001).