ABSTRACT
To study vaccine-induced neutralizing antibodies against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants isolated in Osaka, Japan, microneutralization tests were performed on serum samples from 32subjects who received a second dose of vaccination, and 10 of those who received the third dose of vaccination. Geometric mean titres (GMTs) for the D614G strain, Alpha variant, Delta variant, and Omicron BA.1 of the subjects after the second dose of vaccination were 19.5, 21.8, 6.3 and 2.0, respectively. The GMT for the Delta variant was significantly lower than that for the D614G strain and Alpha variant, and the GMT for the Omicron BA.1 was significantly lower than that for the Delta variant. Among the subjects who received three doses of vaccination, the GMTs for the Omicron BA.1 (62.8) and BA.2 (38.6) were significantly higher than that for the Omicron BA.1 after the second dose. Thus, in the present study, the second dose of vaccination induced neutralizing antibodies against SARS-CoV-2 strains, and the reactivity of neutralizing antibodies to the variants was thought to be enhanced by the third dose of vaccination. The serum samples used in this study will be useful in evaluating the reactivity of vaccine-induced antibodies to newly emerging variants.
ABSTRACT
Since the coronavirus disease 2019 (COVID-19) outbreak, laboratory diagnosis has mainly been conducted using reverse-transcription polymerase chain reaction (RT-PCR). Detecting the presence of an infectious virus in the collected sample is essential to analyze if a person can transmit infectious severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). However, there have been no quantitative investigations conducted for infectious SARS-CoV-2 in clinical samples. Therefore, in the present study, a rapid and simple focus-forming assay using the peroxidase-antiperoxidase technique was developed to quantify infectious SARS-CoV-2 titers in 119 samples (n = 52, nasopharyngeal swabs [NPS]; n = 67, saliva) from patients with COVID-19. Furthermore, the study findings were compared with the cycle threshold (Ct) values of real-time RT-PCR. The infectious virus titers in NPS samples and Ct values were inversely correlated, and no infectious virus could be detected when the Ct value exceeded 30. In contrast, a low correlation was observed between the infectious virus titers in saliva and Ct values (r = -0.261, p = 0.027). Furthermore, the infectious virus titers in the saliva were significantly lower than those in the NPS samples. Ten days after the onset of COVID-19 symptoms, the infectious virus was undetectable, and Ct values were more than 30 in NSP and saliva samples. The results indicate that patients whose symptoms subsided 10 days after onset, with Ct values more than 30 in NSP and saliva samples, were less likely to infect others.