Background/Objectives: Millions of individuals worldwide continue to experience symptoms following SARS-CoV-2 infection. This study aimed to assess the prevalence and phenotype of multi-system symptoms attributed to Long COVID—including fatigue, pain, cognitive-emotional disturbances, headache, cardiopulmonary issues, and alterations in taste and smell—that have persisted for at least two years after acute infection, which we define as “persistent Long COVID”. Additionally, the study aimed to identify clinical features and blood biomarkers associated with persistent Long COVID symptoms. Methods: We sent a detailed long COVID symptoms questionnaire to an existing cohort of 1258 vaccinated adults (age 18–79 years) who had mild infection (e.g., non-hospitalized) SARS-CoV-2 Delta variant 2 years earlier. These individuals had comprehensive datasets, including blood samples, available for further analysis. We estimated prevalence of persistent long COVID two years post-infection using weighted adjustment (Horvitz–Thompson estimator) to overcome reporting bias. Multivariable logistic regression models were used to determine association of clinical features and blood biomarkers (pre-infection SARS-CoV-2 RBD-IgG, SARS-CoV-2 neutralizing antibodies, and pre-infection and post-infection neurofilament light) with prevalence of persistent long COVID. Results: N = 323 participants responded to the survey, of whom N = 74 (23%) reported at least one long COVID symptom that had persisted for two years after the acute infection. Weighted prevalence of persistent long COVID symptoms was 21.5% (95% CI = 16.7–26.3%). Female gender, smoking, and severity of acute COVID-19 infection were significantly associated with persistent Long COVID. The blood biomarkers assessed were not significantly associated with persistent Long COVID. Conclusions: Among vaccinated adults two years after mild infection with Delta variant SARS-CoV-2, persistent symptoms attributed to Long COVID are extremely common, certain subgroups are at higher risk, and further research into biological mechanisms and potential treatment targets is needed.

The immune response of liver transplant (LT) recipients to a third dose of the BNT162b2 mRNA vaccine significantly waned after four months. We aimed to evaluate the immune response and breakthrough infection rates of a fourth dose against the Omicron variants among LT recipients. LT recipients who had no past or active SARS-CoV-2 infection and received three doses of the BNT162b2mRNA vaccine were included. Of the 73 LT recipients, 50 (68.5%) received a fourth dose. The fourth dose was associated with a significantly higher positive immune response than the third dose. Receptor-binding domain (RBD) IgG and Omicron BA.1 and BA.2 neutralizing antibodies were determined at a median of 132 and 29 days after the third and fourth vaccines. They were 345 binding antibody units per milliliter (BAU/mL) vs. 2118 BAU/mL (p < 0.0001), 10 vs. 87 (p < 0.0001), and 15 vs. 149 (p = 0.001), respectively. Breakthrough infections were documented among nine (18%) LT recipients after the fourth dose and among seven (30.4%) patients following the third dose (p = 0.2); 93.5% of breakthrough infections were mild. The infection rate after the fourth dose was higher among diabetic vs. nondiabetic recipients (33.3% vs. 6.9%, respectively; p = 0.02). Further studies are needed to evaluate additional factors influencing the breakthrough infection rate among LT recipients.

To study the differences in the immune response to SARS-CoV-2 infection compared to the response to vaccination, we characterized the humoral immune kinetics of these situations. In this prospective longitudinal study, we followed unvaccinated COVID-19-recovered individuals (n = 130) and naïve, two-dose BNT162b2-vaccinated individuals (n = 372) who were age- and BMI-matched for six months during the first pandemic year. Anti-RBD-IgG, neutralizing antibodies (NAbs), and avidity were assessed monthly. For recovered patients, data on symptoms and the severity of the disease were collected. Anti-RBD-IgG and NAbs titers at peak were higher after vaccination vs. after infection, but the decline was steeper (peak log IgG: 3.08 vs. 1.81, peak log NAbs: 5.93 vs. 5.04, slopes: −0.54 vs. −0.26). Peak anti-RBD-IgG and NAbs were higher in recovered individuals with BMI > 30 and in older individuals compared to individuals with BMI < 30, younger population. Of the recovered, 42 (36%) experienced long-COVID symptoms. Avidity was initially higher in vaccinated individuals compared with recovered individuals, though with time, it increased in recovered individuals but not among vaccinated individuals. Here, we show that while the initial antibody titers, neutralization, and avidity are lower in SARS-CoV-2-recovered individuals, they persist for a longer duration. These results suggest differential protection against COVID-19 in recovered-unvaccinated vs. naïve-vaccinated individuals.

There are limited data concerning the immunogenicity and reactogenicity of COVID-19 vaccines in children. A total of 110 children, 5–11 years old were vaccinated with two doses (with a 3-week interval between doses) of the Pfizer-BioNTech COVID-19 vaccine and were followed for 21, 90, and 180 days after vaccination for immunogenicity, adverse events, and breakthrough infections. Ninety days after the first vaccine dose, the GeoMean (CI 95%) of IgG ascended to 1291.0 BAU (929.6–1790.2) for uninfected children and 1670.0 BAU (1131.0–2466.0) for Infected children. One hundred and eighty days after receiving the first dose of the vaccine, the titers decreased to 535.5 BAU (288.4–993.6) for the uninfected children, while only a small decline was detected among infected children—1479.0 (878.2–2490.0). The neutralizing antibodies titer almost did not change over time in the uninfected children, and even elevated for the infected children. Of the 110 vaccinated children, 75.5% were infected, with only mild COVID-19 infection symptoms. Child vaccination was found to be safe, with mild, mostly local, and of short duration, reported AEs. No serious adverse events (SAEs) were reported after vaccination. The durability of two doses of vaccine in children is longer, thus a booster may not be needed as early as in adults.

Background/Objectives: During the 2025–26 influenza season, the antigenically distinct A(H3N2) subclade K emerged and spread globally, raising significant concerns regarding seasonal vaccine effectiveness. This study aimed to map antigenic and structural divergence of this novel strain compared to the 2025/26 A(H3N2) vaccine component. Furthermore, we investigated how this vaccine mismatch impacts functional humoral immunity by directly comparing antibody neutralization profiles between individuals with vaccine-induced protection and those with infection-acquired immunity. Methods: Phylogenetic and structural modeling analyses were performed on 2025–26 Israeli influenza samples to identify genetic distances and mutations in the hemagglutinin protein compared to the 2025/26 A(H3N2) vaccine component. Hemagglutination inhibition (HI) and microneutralization assays were conducted using 2025–26 winter serum samples from a cohort of vaccinated healthcare workers and vaccinated convalescents to quantify antibody responses against both vaccine-like and subclade K strains. Results: Phylogenetic analysis revealed a clear dominance of the subclade K strain in Israel during the winter of 2025–26. Significant vaccine mismatch was defined by eight newly identified hemagglutinin mutations in the circulating subclade K strains. Serological testing demonstrated that while vaccination alone produced a modest two-to-three-fold increase in antibody titers, baseline protection against subclade K remained critically low. Conversely, natural infection in vaccinated convalescents elicited a significantly stronger, broader immune response, demonstrating a 6-fold and 13-fold increase in neutralizing antibodies against the vaccine-like and subclade K strains, respectively. Conclusions: Natural infection provides a more pronounced increase in both binding and neutralizing antibodies compared to vaccination alone, which might contribute to broader cross-protection against antigenically drifted variants. These findings highlight the primary limitations of current influenza strain prediction and emphasize the urgent need for faster, more adaptable vaccine manufacturing platforms to better match actual circulating strains.

West Nile virus (WNV) is a mosquito-borne pathogen of growing public health concern that is endemic in Israel and has been responsible for intermittent outbreaks. The seroprevalence and durability of functional antibody responses in asymptomatic WNV-infected populations remain unclear. We screened a large cohort of asymptomatic healthcare workers (HCWs) for WNV IgG and neutralizing antibody titers. Predictors of seropositivity and neutralization levels were assessed using multivariable regression. Among seropositive individuals, longitudinal samples, collected up to 44 months prior to the final blood sampling, were analyzed to evaluate neutralizing antibody kinetics. Of 2,365 HCWs screened (median age 47 years; 66.6% females), 14.8% (n=301) were WNV IgG seropositive, and 94.0% of these demonstrated detectable neutralizing efficacy. Age was independently associated with IgG seropositivity (adjusted OR 1.33 per 10 years; 95% CI: 1.23-1.45). Neutralizing efficacy was high (Geometric-Mean-Titer 6171, 95% CI 4361-8723), and IgG and neutralizing antibody titers were correlated (R²=0.17). Over 44 months, a 23% annual decrease in neutralizing titers was observed. WNV seropositivity increases with age, indicating cumulative exposure. Correlation between IgG and neutralizing efficacy suggests durable protection among seropositive individuals. Despite waning antibody levels, high neutralizing capacity likely confers extended protection.