A total of 32 Omicron-infected patients, ranging from 14 to 63 years old, with a mean age of 36 years old, were identified, 12 of whom were male. Nineteen infected patients received inactivated vaccine (BBIBP-CorV or CoronaVac), of which 13 received three doses of inactivated vaccine and six received two doses of inactivated vaccine (Table
1). Among these 32 infected patients, both IgM and IgG antibodies specific to the RBD protein of Wuhan and Omicron strains were at undetectable levels within 7 days of symptom onset, the titers of Omicron-specific IgM antibodies were significantly elevated after day 7 of the onset, peaking at 14–21 days after onset. However, no significant increase in the titers of Wuhan-specific IgM levels was observed within 27 days of onset (Fig.
1A). On the other hand, 40 sera from individuals infected with Wuhan virus strain were used as controls, it was found that Wuhan strain-specific IgM antibodies were present in sera within 27 days after onset, in contrast, within 27 days after onset, Omicron-specific IgM has not been obviously detected (Fig.
1B). Since most of the patients had been vaccinated against COVID-19, most of them had detectable IgG titers against Wuhan and Omicron strains within 7 days of symptom onset, the specific IgG titer against Wuhan strain increased significantly within 8–14 days of symptom onset and peaked within 14 days, in contrast, the specific IgG titer against Omicron strain did not elevate significantly during the observation (Fig.
1C). Specifically, for individuals who have been vaccinated against COVID-19, the titers of IgM against Wuhan strain, IgG against Wuhan strain, and Omicron-specific IgG have no significant difference between the early and later stages of symptom onset (Fig.
1D). Similarly, for unvaccinated individuals, only Omicron-specific IgM titers were significantly elevated with time delay in the time range of the assay (Fig.
1E). For overall patients with COVID-19, Wuhan strain-specific IgG and Omicron-specific IgM antibody titers are significantly increased at the later stage of the disease, while other antibody indicators do not change significantly during different stages (Fig.
1F). In addition, we tested the titers of neutralizing antibodies against Wuhan and Omicron strains in COVID-19 patients at different stages after onset. In the early stages of onset (0 day − 7th day after onset), neutralizing antibodies against the Wuhan strain were detected in each vaccinated individual (geometric mean titer [GMT] 73.0 [4.0-1024.0]), and the GMT was 30.5 (4.0-1024.0) against Omicron-BA.2 strain. In the late stages of infection (8th day-27th day after onset), serums from Omicron breakthrough infected individuals had significantly higher antibodies against the Wuhan and Omicron strains when compared to those in the early stages, with a mean of GMT 585.0 (32.0-5142.0) against Wuhan strain and GMT 118.4 against Omicron strain (8.0-512.0). The titer of specific neutralizing antibodies against Wuhan strain was significantly higher than that of specific neutralizing antibody against Omicron at both early and late stages after onset (Fig.
1G). While among the 13 unvaccinated individuals, only the sera from three infected individuals (3/13) showed neutralization activity against the Wuhan strain with a GMT 16 (8.0–16.0), and sera from four infected individuals (4/13) revealed neutralization activity against the Omicron strain with GMT 18.1 (4.0-128.0) (Fig.
1H). In the later stage after onset (8th day-27th day of infection), neutralizing antibody titers against Wuhan and Omicron strains did not increase significantly when compared to those in the early stage, sera from three infected individuals (3/13) had neutralizing potency against the Wuhan strain (GMT 19.5), and all unvaccinated infected individuals produced neutralizing antibodies against Omicron at the late stage (GMT 14.1) (Fig.
1I).