Large-scale cross-sectional seroepidemiologic study of COVID-19 in Japan: Acquisition of herd immunity and the vaccines' efficacy

Background: The COVID-19 pandemic situation has been changing drastically worldwide due to the continuous appearance of SARS-CoV-2 variants and the roll-out of mass vaccination. Periodic cross-sectional studies during the surge of COVID-19 cases is essential to elucidate the pandemic situation. Methods: Sera of 1,000 individuals who underwent a health check-up in Hyogo Prefecture Health Promotion Association clinics in Japan were collected in August and December 2021. Antibodies against SARS-CoV-2 N and S antigens were detected in the sera by an electrochemiluminescence immunoassay (ECLIA) and an enzyme-linked immunosorbent assay (ELISA), respectively. The sera's neutralization activities for the conventional SARS-CoV-2 (D614G), Delta, and Omicron variants were measured. Results: The seropositive rates for the antibody against N antigen were 2.1% and 3.9% in August and December 2021 respectively, demonstrating a Delta variant endemic during that time; the actual infection rate was approximately twofold higher than the rate estimated based on the polymerase chain reaction (PCR)-based diagnosis. The anti-S seropositive rate was 38.7% in August and it reached 90.8% in December, in concordance with the vaccination rate in Japan. In the December cohort, 78.7% of the sera showed neutralizing activity against the Delta variant, whereas that against the Omicron was much lower at 36.6%. Conclusions: These analyses revealed that herd immunity against SARS-CoV-2 including the Delta variant was established in December 2021, leading to convergence of the variants. The low neutralizing activity against the Omicron variant suggests the need for the further promotion of the prompt three-dose vaccination to overcome this variant's imminent 6th wave in Japan.

Two Opposing Roles of SARS-CoV-2 RBD-Reactive Antibodies in Pre-Pandemic Plasma Samples From Elderly People in ACE2-Mediated Pseudovirus Infection

A novel coronavirus designated severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) emerged and caused an outbreak of unusual viral pneumonia. Several reports have shown that cross-reactive antibodies against SARS-CoV-2 also exist in people unexposed to this virus. However, the neutralizing activity of cross-reactive antibodies is controversial. Here, we subjected plasma samples from SARS-CoV-2-unexposed elderly Korean people (n = 119) to bead-based IgG antibody analysis. SARS-CoV-2 S1 subunit-reactive IgG antibody analysis detected positive signals in some samples (59 of 119, 49.6%). SARS-CoV-2 receptor-binding domain (RBD)-reactive antibody levels were most significantly correlated with human coronavirus-HKU1 S1 subunit-reactive antibody levels. To check the neutralizing activity of plasma samples, the SARS-CoV-2 spike pseudotype neutralizing assay was used. However, the levels of cross-reactive antibodies did not correlate with neutralizing activity. Instead, SARS-CoV-2 pseudovirus infection was neutralized by some RBD-reactive plasma samples (n = 9, neutralization ≥ 25%, P ≤ 0.05), but enhanced by other RBD-reactive plasma samples (n = 4, neutralization ≤ -25%, P ≤ 0.05). Interestingly, the blood plasma groups with enhancing and neutralizing effects had high levels of SARS-CoV-2 RBD-reactive antibodies than the plasma group that had no effect. These results suggest that some SARS-CoV-2 RBD-reactive antibodies from pre-pandemic elderly people exert two opposing functions during SARS-CoV-2 pseudovirus infection. In conclusion, preformed RBD-reactive antibodies may have two opposing functions, namely, protecting against and enhancing viral infection. Analysis of the epitopes of preformed antibodies will be useful to elucidate the underlying mechanism.

Delta infection following vaccination elicits potent neutralizing immunity against the SARS-CoV-2 Omicron

Since the initial detection of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Omicron variant (B.1.1.529) in November 2021 in South Africa, it has caused a rapid increase in infections globally. The Omicron variant encodes 37 amino acid substitutions in its spike protein, and early reports have provided evidence for extensive immune escape and reduced vaccine effectiveness. We assessed serum neutralizing activity in sera from Delta infection following vaccination of CoronaVac or ZF2001 and Delta infection only against SARS-CoV-2 Wuhan-Hu-1 (WA1), Beta, Delta, and Omicron. We found that sera from Delta infection only could neutralize WA1 and Delta pseudoviruses but nearly completely lost capacity to neutralize Beta and Omicron pseudoviruses. However, Delta infection following vaccination resulted in a significant increase of serum neutralizing activity against WA1, Beta, and Omicron. This study demonstrates that breakthrough infection of Delta in previously vaccinated individuals substantially induced high potency humoral immune response against the Omicron variant and other emerged variants.

Older Adults Mount Less Durable Humoral Responses to a Two-dose COVID-19 mRNA Vaccine Regimen, but Strong Initial Responses to a Third Dose

Background. Two-dose mRNA vaccines reduce COVID-19 related hospitalization and mortality, but immune protection declines over time. As such, third vaccine doses are now recommended, particularly for older adults. We examined immune response durability up to 6 months after two vaccine doses, and immunogenicity after a third vaccine dose, in 151 adults ranging in age from 24 to 98 years. Methods. Specimens were collected from 81 healthcare workers (median age 41 years), 56 older adults (median 78 years) and 14 COVID-19 convalescent individuals (median 48 years), at one, three and six months following the second dose, and from 15 HCW, 28 older adults and 3 convalescent individuals at one month following a third dose. Binding antibodies to the SARS-CoV-2 spike receptor binding domain were quantified using a commercial immunoassay. Virus neutralizing activity was assessed using a live SARS-CoV-2 infection assay. Results. Compared to healthcare workers, older adults displayed ~0.3 log10 lower peak binding antibodies one month after the second dose (p<0.0001) and modestly faster rates of antibody decline thereafter (p=0.0067). A higher burden of chronic health conditions was independently associated with faster rates of antibody decline after correction for age, sociodemographic factors, and vaccine-related variables. Peak neutralizing activity was 4-fold lower in older adults one month after the second dose (p<0.0001) and became undetectable in the majority of individuals by six months. One month after a third dose, binding antibodies and neutralizing activities surpassed peak values achieved after two doses in both healthcare workers and older adults, and differences between these groups were no longer statistically significant. Compared to both naive groups, convalescent individuals displayed slower rates of binding antibody decline (p<0.006) and maintained higher neutralizing activity six months after the second dose. Conclusions. Immune responses to two-dose COVID-19 mRNA vaccines are overall weaker in older adults, and also decline more quickly over time, compared to younger adults. A third COVID-19 mRNA vaccine dose enhanced binding and neutralizing antibodies to levels higher than those observed after two vaccine doses, but the rate of decline of these responses should be monitored, particularly in older adults with a higher burden of chronic health conditions.

Immune responses to inactivated and vector-based vaccines in individuals previously infected with SARS-CoV-2

Immunity wanes in individuals previously infected with SARS-CoV-2, and vaccinating those individuals may help reduce reinfection. Herein, reactogenicity and immunogenicity following vaccination with inactivated (CoronaVac) and vector-based (ChAdOx1-S, AZD1222) vaccines were examined in previously infected individuals. Immune response was also compared between short and long intervals between first date of detection and vaccination. Adverse events were mild but were higher with AZD1222 than with CoronaVac. Baseline IgG-specific antibodies and neutralizing activity were significantly higher with shorter than longer intervals. With a single-dose vaccine, IgG and IgA-specific binding antibodies, neutralizing activity, and total interferon-gamma response peaked at 14 days. Immune response was significantly higher in recovered individuals than in infection-naïve individuals. Antibody response was greater with longer than shorter intervals. AZD1222 induced higher antibody and T cell responses than those of CoronaVac. Thus, to achieve immunity, individuals with prior SARS-CoV-2 exposure may require only a single dose of AZD1222 or two doses of CoronaVac to achieve the immune response. These findings supported vaccine strategies in previously infected individuals.

Non-neutralizing antibodies targeting the immunogenic regions of HIV-1 envelope reduce mucosal infection and virus burden in humanized mice

Antibodies are principal immune components elicited by vaccines to induce protection from microbial pathogens. In the Thai RV144 HIV-1 vaccine trial, vaccine efficacy was 31% and the sole primary correlate of reduced risk was shown to be vigorous antibody response targeting the V1V2 region of HIV-1 envelope. Antibodies against V3 also were inversely correlated with infection risk in subsets of vaccinees. Antibodies recognizing these regions, however, do not exhibit potent neutralizing activity. Therefore, we examined the antiviral potential of poorly neutralizing monoclonal antibodies (mAbs) against immunodominant V1V2 and V3 sites by passive administration of human mAbs to humanized mice engrafted with CD34+ hematopoietic stem cells, followed by mucosal challenge with an HIV-1 infectious molecular clone expressing the envelope of a tier 2 resistant HIV-1 strain. Treatment with anti-V1V2 mAb 2158 or anti-V3 mAb 2219 did not prevent infection, but V3 mAb 2219 displayed a superior potency compared to V1V2 mAb 2158 in reducing virus burden. While these mAbs had no or weak neutralizing activity and elicited undetectable levels of antibody-dependent cellular cytotoxicity (ADCC), V3 mAb 2219 displayed a greater capacity to bind virus- and cell-associated HIV-1 envelope and to mediate antibody-dependent cellular phagocytosis (ADCP) and C1q complement binding as compared to V1V2 mAb 2158. Mutations in the Fc region of 2219 diminished these effector activities in vitro and lessened virus control in humanized mice. These results demonstrate the importance of Fc functions other than ADCC for antibodies without potent neutralizing activity.

Functional Analysis of Human and Feline Coronavirus Cross-Reactive Antibodies Directed Against the SARS-CoV-2 Fusion Peptide

To face the continuous emergence of SARS-CoV-2 variants, broadly protective therapeutic antibodies are highly needed. We here focused on the fusion peptide (FP) region of the viral spike antigen since it is highly conserved among alpha- and betacoronaviruses. First, we found that coronavirus cross-reactive antibodies are commonly formed during infection, being omnipresent in sera from COVID-19 patients, in ~50% of pre-pandemic human sera (rich in antibodies against endemic human coronaviruses), and even in feline coronavirus-infected cats. Pepscan analyses demonstrated that a confined N-terminal region of the FP is strongly immunogenic across diverse coronaviruses. Peptide-purified human antibodies targeting this conserved FP epitope exhibited broad binding of alpha- and betacoronaviruses, besides weak and transient SARS-CoV-2 neutralizing activity. Being frequently elicited by coronavirus infection, these FP-binding antibodies might potentially exhibit Fc-mediated effector functions and influence the kinetics or severity of coronavirus infection and disease.

Human Milk SARS-CoV-2 Antibodies up to 6 Months After Vaccination

BACKGROUND: Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)–specific antibodies have been detected in human milk up to 6 weeks post–coronavirus disease 2019 (COVID-19) vaccination. We evaluated SARS-CoV-2-specific antibodies, neutralization activity, effect of pasteurization, and persistence through 6 months after vaccination. METHODS: This prospective longitudinal study enrolled 30 pregnant or lactating women. SARS-CoV-2 antibodies and neutralization capacity were analyzed using an enzyme-linked immunosorbent assay compared at prevaccination and 1, 3, and 6 months postvaccination, and through Holder pasteurization. RESULTS: Human milk SARS-CoV-2-specific IgG levels peaked at 1 month postvaccination and persisted above prevaccination levels for at least 6 months (P = .005). SARS-CoV-2-specific IgA was detected at 1 and 3 months (both P &lt; .001) but waned by 6 months compared with baseline (P = .07). Milk SARS-CoV-2-specific IgG and IgA correlated with serum IgG at the same time point (R2 = 0.37, P &lt; .001 and R2 = 0.19, P &lt; .001). Neutralization activity was seen in 83.3%, 70.4%, and 25.0% of milk samples at 1, 3, and 6 months postvaccination. Neutralization most strongly correlated with SARS-CoV-2-specific IgG (R2 = 0.57, P &lt; .001). Pre- and postpasteurization samples showed similar IgG (0.84 vs 1.07, P = .36) and neutralizing activity (57.7% vs 58.7% inhibition, P = .27), but lower IgM and IgA levels postpasteurization (0.09 vs 0.06, P = .004 and 0.21 vs 0.18, P = .043). CONCLUSIONS: The data suggest that human milk SARS-CoV-2-specific antibodies may be available to milk-fed infants for up to 6 months. In addition, donor milk from vaccinated mothers retain IgG and neutralizing activity.

Mapping the antigenic diversification of SARS-CoV-2

Large-scale vaccination campaigns have prevented countless SARS-CoV-2 infections, hospitalizations and deaths. However, the emergence of variants that escape from immunity challenges the effectiveness of current vaccines. Given this continuing evolution, an important question is when and how to update SARS-CoV-2 vaccines to antigenically match circulating variants, similar to seasonal influenza viruses where antigenic drift necessitates periodic vaccine updates. Here, we studied SARS-CoV-2 antigenic drift by assessing neutralizing activity against variants-of-concern (VOCs) of a unique set of sera from patients infected with a range of VOCs. Infections with ancestral or Alpha strains induced the broadest immunity, while individuals infected with other VOCs had more strain-specific responses. Omicron was substantially resistant to neutralization by sera elicited by all other variants. Antigenic cartography revealed that all VOCs preceding Omicron belong to one antigenic cluster, while Omicron forms a new antigenic cluster associated with immune escape and likely requiring vaccine updates to ensure vaccine effectiveness.