National seroepidemiological study of COVID-19 after the initial rollout of vaccines: Before and at the peak of the Omicron-dominant period in Japan

SARS-CoV-2 seroprevalence among healthcare workers in a highly vaccinated Japanese medical center from 2020-2023

Infection-induced SARS-CoV-2 seroprevalence has been studied worldwide. At Juntendo University Hospital (JUH) in Tokyo, Japan, we have consistently conducted serological studies using the blood residue of healthcare workers (HCWs) at annual health examinations since 2020. In this 2023 study (n = 3,594), N-specific seroprevalence (infection-induced) was examined while univariate and multivariate logistic regression analyses were performed to compute ORs of seroprevalence with respect to basic characteristics of participants. We found that the N-specific seroprevalence in 2023 was 54.1%-a jump from 17.7% in 2022, and 1.6% in 2021-with 37.9% as non-PCR-confirmed asymptomatic infection cases. Those younger than 50 (adjusted OR = 1.62; p < .001) and recipients with 4 doses or less of vaccine had a higher risk to be N-positive, ranging from 1.45 times higher for the participants with 4 doses (p < .001) to 4.31 times higher for the participants with 1 dose (p < .001), compared to those with 5 or more doses. Our findings indicate that robust vaccination programs may have helped alleviate symptoms but consequently caused asymptomatic spread in this hospital, especially among younger HCWs. Although having four doses or less was found to be associated with higher risk of infection, the optimal constitution and intervals for effective booster vaccines warrant further investigations.

Infection- and Vaccine-Induced SARS-CoV-2 Seroprevalence, Japan, 2023

We assessed SARS-CoV-2 seroprevalence in Japan during July-August 2023, with a focus on 2 key age groups, 0-15 and >80 years. We estimated overall seroprevalence of 45.3% for nucleocapsid antibodies and 95.4% for spike antibodies and found notable maternally derived spike antibodies in infants 6-11 months of age (90.0%).

Prevalence of SARS-CoV-2-specific antibodies in a sample of the Lithuanian population-based study in Spring 2023

Objectives

Despite positive trends in SARS-CoV-2 epidemiology, seroprevalence surveys remain an important tool for estimating the magnitude of the COVID-19 pandemic. This study aimed to investigate the prevalence of IgG antibodies against SARS-CoV-2 nucleocapsid (N) and spike (S) proteins in a sample of the Lithuanian population (N = 517) and evaluate how the pattern of seropositivity correlates with the levels of SARS-CoV-2 infection and vaccination.

Methods

Study participants (aged 18-88 years) filled in the questionnaire self-reporting their demographic-social variables, health status, and SARS-CoV-2-related status. The anti-S and anti-N IgG levels were estimated using a microarray ELISA test.

Results

After several pandemic waves and vaccination campaign, the seroprevalence of SARS-CoV-2-specific IgG in the analyzed sample was 97.87 % by March-May 2023. We determined the 96.91 % prevalence of anti-S and 58.03 % prevalence of anti-N IgG. The majority of study participants (71.18 %) had hybrid immunity induced by vaccination and SARS-CoV-2 infection. 20.3 % of study participants were anti-N IgG positive without reporting any previous symptoms or a positive SARS-CoV-2 test. A decline of anti-N IgG positivity within 9 months after infection was observed.

Conclusions

This study demonstrates high total seroprevalence in March-May 2023 in all age groups indicating a widely established humoral immunity against SARS-CoV-2 in Lithuania.

Infection-mediated immune response in SARS-CoV-2 breakthrough infection and implications for next-generation COVID-19 vaccine development

Post-vaccination infections, termed breakthrough infections, occur after the virus infection overcomes the vaccine-induced immune barrier. During the early stages of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Omicron wave, high serum-neutralizing antibody titers against the Omicron variant were detected in individuals with breakthrough infections as well as those who received a third vaccine dose (i.e., booster recipients). Additionally, these cases indicated that Omicron antigens triggered an immune response that differed from that triggered by the vaccine strain before analysis of the effectiveness of new vaccines updated for the Omicron variants. Moreover, the magnitude and breadth of neutralizing antibody titers induced by breakthrough infections are correlated with the upper respiratory viral load at diagnosis and the duration between vaccination and infection, respectively. Unlike booster vaccine recipients, patients with breakthrough infections have varying durations between vaccination and infection. Accordingly, optimal booster vaccination intervals may be estimated based on the cross-neutralizing antibody response induced over time. Examination of breakthrough infection cases has provided valuable insights that could not be yielded by only examining vaccinated individuals alone. These insights include estimates of vaccine-induced immunity against SARS-CoV-2 variants and the various factors related to the clinical status. This review describes the immune response elicited by breakthrough infections; specifically, it discusses factors that affect the magnitude and breadth of serum antibody titers as well as the appropriate booster vaccination strategy. This review provides key aspects that could contribute to developing next-generation COVID-19 vaccines through breakthrough infection cases.

Seroprevalence of severe acute respiratory syndrome coronavirus 2 N antibodies between December 2021 and march 2023 in Japan

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) emerged in 2019 in China and rapidly spread worldwide, leading to a pandemic. The threat of SARS-CoV-2 is subsiding as most people have acquired sufficient antibodies through vaccination and/or infection to prevent severe COVID-19. After the emergence of the omicron variants, the seroprevalence of antibodies against the N protein elicited by SARS-CoV-2 infection ranged from 44.4% to 80.2% in countries other than Japan. Here, we assessed the seroprevalence in Japan before and after the appearance of omicron variants. Serosurveillance of antibodies against N was conducted between December 2021 and March 2023 in Japan. In total, 7604 and 3354 residual serum or plasma samples were collected in the Tokyo metropolitan area and Sapporo, respectively. We found that the seroprevalence in representative regions of Japan increased approximately 3% to 23% after the emergence of the omicron variants. We also found higher seroprevalence among the young compared with the elderly. Our findings indicate that unlike other countries, most of the Japanese population has not been infected, raising the possibility of future SARS-CoV-2 epidemics in Japan.

Protective effect of previous infection and vaccination against reinfection with BA.5 Omicron subvariant: a nationwide population-based study in Japan

Background

The Omicron variant of SARS-CoV-2 was reported to evade immunity derived from vaccination and previous infection. A better understanding of hybrid immunity informs effective infection control strategies. Since the reinfection risk was not well-assessed in East Asia, this study aims to evaluate the risk of infection with Omicron subvariant BA.5 among previously infected individuals in Japan.

Methods

All notified cases were extracted from the Japanese national COVID-19 surveillance database including 20,297,335 records up to 25 September 2022. Reinfection with BA.5 was defined as the infection notified during the BA.5 dominated period with any prior SARS-CoV-2 infection. The protective effect of prior infections against reinfections with BA.5 was estimated by applying a case-population design and the protective effect of vaccination was estimated by a multivariable Cox regression adjusting for age, sex, variants of prior infection, and the time since the last vaccination.

Findings

Among 19,830,548 SARS-CoV-2 first infections, 233,424 (1.2%) were reinfected with BA.5. The protective effect against BA.5 reinfection of prior infection with Wuhan strain was 46%, Alpha variant was 35%, Delta variant was 41%, and BA.1/BA.2 subvariant was 74%. The reduced risk of BA.5 reinfection by 7%, 33%, and 66% was associated with two, three, and four doses of vaccination, respectively, compared with one-dose vaccination.

Interpretation

The prior infections with Omicron subvariant BA.1/BA.2 protected BA.5 reinfection more than pre-Omicron variants. Increased frequency of vaccination led to more protection from reinfection with BA.5. Up-to-date vaccination may be encouraged to prevent future reinfection among the previously infected population.

Funding

None.

Immune escape and waning immunity of COVID-19 monovalent mRNA vaccines against symptomatic infection with BA.1/BA.2 and BA.5 in Japan

BACKGROUND

Repeated emergence of variants with immune escape capacity and waning immunity from vaccination are major concerns for COVID-19. We examined whether the surge in Omicron subvariant BA.5 cases was due to immune escape or waning immunity through vaccine effectiveness (VE) evaluation.

METHODS

A test-negative case-control study was conducted in 16 clinics/hospitals during the BA.1/BA.2-dominant and BA.5-dominant periods. VE against symptomatic infection was estimated after adjusting for age, sex, comorbidity, occupation, testing frequency, prior infection, close contact history, clinic/hospital, week, and preventive measures. Absolute VE (aVE) was calculated for 2/3/4 doses, compared to the unvaccinated. Relative VE (rVE) was calculated, comparing 3 vs 2 and 4 vs 3 doses.

RESULTS

13,025 individuals were tested during the BA.1/BA.2-dominant and BA.5-dominant periods with similar baseline characteristics. For BA.1/BA.2, aVE was 52 % (95 %CI:34-66) 14 days-3 months post-dose 2, 42 % (29-52) > 6 months post-dose 2, 71 % (64-77) 14 days-3 months post-dose 3, and 68 % (52-79) 3-6 months post-dose 3. rVE was 49 % (38-57) 14 days-3 months post-dose 3 and 45 % (18-63) 3-6 months post-dose 3. For BA.5, aVE was 56 % (27-73) 3-6 months post-dose 2, 32 % (12-47) > 6 months post-dose 2, 70 % (61-78) 14 days-3 months post-dose 3, 59 % (48-68) 3-6 months post-dose 3, 50 % (29-64) > 6 months post-dose 3, and 74 % (61-83) ≥ 14 days post-dose 4. rVE was 56 % (45-65) 14 days-3 months post-dose 3, 39 % (27-48) 3-6 months post-dose 3, 25 % (-2-45) > 6 months post-dose 3, and 30 % (-6-54) ≥ 14 days post-dose 4.

CONCLUSIONS

Booster doses initially provided high protection against BA.5 at a level similar to that against BA.1/BA.2. However, the protection seemed shorter-lasting against BA.5, which likely contributed to the surge. Furthermore, rVE post-dose 4 was low even among recent vaccinees. These results support the introduction of variant-containing vaccines and emphasize the need for vaccines with longer duration of protection.

Bivalent Vaccine Effectiveness Among Adults Aged ≥65 Years During the BA.5-Predominant Period in Japan: The VENUS Study

Background

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Omicron BA.5 became prevalent in July 2022 in Japan. Bivalent messenger RNA (mRNA) vaccines were approved as booster doses for individuals who received the primary series or booster dose by monovalent vaccines. We aimed to assess the effectiveness of bivalent vaccines in Japanese adults aged ≥65 years.

Methods

We conducted a population-based cohort study using data collected from January 2019 to February 2023 in Japan. We included individuals aged ≥65 years in a municipality who received the first or second booster dose of monovalent mRNA vaccines. We estimated the effectiveness of the second or third booster dose of bivalent mRNA vaccines during the Omicron BA.5-predominant period (July-December 2022), compared with ≥90 days after the booster dose of monovalent vaccines. We used a Cox proportional hazard regression model with vaccination status as a time-dependent covariate.

Results

A total of 81 977 individuals aged ≥65 years (mean [standard deviation] age, 78.3 [7.4] years; 33 487 male [40.8%]) were included in the study cohort. Among them, 57 396 were vaccinated with the second or third dose of bivalent vaccines (BA.1 or BA.4/5). The effectiveness against coronavirus disease 2019 (COVID-19) was estimated to be 57.9% (95% confidence interval, 52.7%-62.5%) for ≥14 days after the second or third bivalent booster dose, compared with 90 days after the first or second monovalent booster dose.

Conclusions

The study showed that the bivalent mRNA vaccines as the second and third doses would provide protection against COVID-19 among adults ≥65 years in Japan.

Infection-Induced SARS-CoV-2 Seroprevalence among Blood Donors, Japan, 2022

A nationwide survey of SARS-CoV-2 antinucleocapsid seroprevalence among blood donors in Japan revealed that, as of November 2022, infection-induced seroprevalence of the population was 28.6% (95% CI 27.6%-29.6%). Seroprevalence studies might complement routine surveillance and ongoing monitoring efforts to provide a more complete real-time picture of COVID-19 burden.

Saturation time of exposure interval for cross-neutralization response to SARS-CoV-2: Implications for vaccine dose interval

Evaluating the serum cross-neutralization responses after breakthrough infection with various SARS-CoV-2 variants provides valuable insight for developing variant-proof COVID-19 booster vaccines. However, fairly comparing the impact of breakthrough infections with distinct epidemic timing on cross-neutralization responses, influenced by the exposure interval between vaccination and infection, is challenging. To compare the impact of pre-Omicron to Omicron breakthrough infection, we estimated the effects on cross-neutralizing responses by the exposure interval using Bayesian hierarchical modeling. The saturation time required to generate saturated cross-neutralization responses differed by variant, with variants more antigenically distant from the ancestral strain requiring longer intervals of 2-4 months. The breadths of saturated cross-neutralization responses to Omicron lineages were comparable in pre-Omicron and Omicron breakthrough infections. Our results highlight the importance of vaccine dosage intervals of 4 months or longer, regardless of the antigenicity of the exposed antigen, to maximize the breadth of serum cross-neutralization covering SARS-CoV-2 Omicron lineages.