The evolution of SARS-CoV-2 seroprevalence in Canada: a time-series study, 2020-2023.

BACKGROUND: During the first year of the COVID-19 pandemic, the proportion of reported cases of COVID-19 among Canadians was under 6%. Although high vaccine coverage was achieved in Canada by fall 2021, the Omicron variant caused unprecedented numbers of infections, overwhelming testing capacity and making it difficult to quantify the trajectory of population immunity. METHODS: Using a time-series approach and data from more than 900 000 samples collected by 7 research studies collaborating with the COVID-19 Immunity Task Force (CITF), we estimated trends in SARS-CoV-2 seroprevalence owing to infection and vaccination for the Canadian population over 3 intervals: prevaccination (March to November 2020), vaccine roll-out (December 2020 to November 2021), and the arrival of the Omicron variant (December 2021 to March 2023). We also estimated seroprevalence by geographical region and age. RESULTS: By November 2021, 9.0% (95% credible interval [CrI] 7.3%-11%) of people in Canada had humoral immunity to SARS-CoV-2 from an infection. Seroprevalence increased rapidly after the arrival of the Omicron variant - by Mar. 15, 2023, 76% (95% CrI 74%-79%) of the population had detectable antibodies from infections. The rapid rise in infection-induced antibodies occurred across Canada and was most pronounced in younger age groups and in the Western provinces: Manitoba, Saskatchewan, Alberta and British Columbia. INTERPRETATION: Data up to March 2023 indicate that most people in Canada had acquired antibodies against SARS-CoV-2 through natural infection and vaccination. However, given variations in population seropositivity by age and geography, the potential for waning antibody levels, and new variants that may escape immunity, public health policy and clinical decisions should be tailored to local patterns of population immunity.

Predicting Infectious Severe Acute Respiratory Syndrome Coronavirus 2 From Diagnostic Samples.

BACKGROUND: Reverse-transcription polymerase chain reaction (RT-PCR) has become the primary method to diagnose viral diseases, including severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). RT-PCR detects RNA, not infectious virus; thus, its ability to determine duration of infectivity of patients is limited. Infectivity is a critical determinant in informing public health guidelines/interventions. Our goal was to determine the relationship between E gene SARS-CoV-2 RT-PCR cycle threshold (Ct) values from respiratory samples, symptom onset to test (STT), and infectivity in cell culture. METHODS: In this retrospective cross-sectional study, we took SARS-CoV-2 RT-PCR-confirmed positive samples and determined their ability to infect Vero cell lines. RESULTS: Ninety RT-PCR SARS-CoV-2-positive samples were incubated on Vero cells. Twenty-six samples (28.9%) demonstrated viral growth. Median tissue culture infectious dose/mL was 1780 (interquartile range, 282-8511). There was no growth in samples with a Ct > 24 or STT > 8 days. Multivariate logistic regression using positive viral culture as a binary predictor variable, STT, and Ct demonstrated an odds ratio (OR) for positive viral culture of 0.64 (95% confidence interval [CI], .49-.84; P < .001) for every 1-unit increase in Ct. Area under the receiver operating characteristic curve for Ct vs positive culture was OR, 0.91 (95% CI, .85-.97; P < .001), with 97% specificity obtained at a Ct of > 24. CONCLUSIONS: SARS-CoV-2 Vero cell infectivity was only observed for RT-PCR Ct < 24 and STT < 8 days. Infectivity of patients with Ct > 24 and duration of symptoms > 8 days may be low. This information can inform public health policy and guide clinical, infection control, and occupational health decisions. Further studies of larger size are needed.

The first case of locally acquired tick-borne Babesia microti infection in Canada.

A child with a complicated medical history that included asplenia acquired an infection with Babesia microti in the summer of 2013 and had not travelled outside of Manitoba. Although the clinical findings were subtle, astute laboratory work helped to reach a preliminary identification of Babesia species, while reference laboratory testing confirmed the diagnosis. Blacklegged ticks (Ixodes scapularis) are known to transmit Borrelia burgdorferi and Anaplasma phagocytophilum in the province; however, the present case represents the first known instance of tick-borne B microti, both in Manitoba and in Canada. The expanding territory of the blacklegged tick increases the relevance of this emerging infection. Clinicians, laboratory medical practitioners and public health officials should be aware of B microti as a potential locally acquired infection in Canada.

Un enfant ayant des antécédents médicaux complexes, qui incluaient une asplénie, a contracté une infection à Babesia microti pendant l'été 2013, sans avoir quitté le Manitoba. Même si les résultats cliniques étaient discrets, un travail de laboratoire astucieux a contribué à l'identification préliminaire d'une espèce de Babesia. Le test du laboratoire de référence a confirmé le diagnostic. On sait que les tiques occidentales à pattes noires (Ixodes scapularis) transmettent le Borrelia burgdorferi et l'Anaplasma phagocytophilum dans la province. Le présent cas est toutefois la première occurrence connue de B microti à tique, tant au Manitoba qu'au Canada. L'expansion du territoire de la tique occidentale à pattes noires renforce la pertinence de cette infection émergente. Les cliniciens, les praticiens de laboratoires médicaux et les directeurs de la santé publique devraient savoir que le B microti peut être transmis localement au Canada.

Pandemic (H1N1) 2009 influenza in hospitalized children in Manitoba: nosocomial transmission and lessons learned from the first wave.

OBJECTIVE: To review the experiences at Winnipeg Children's Hospital (WCH) during the 2009 influenza season, with an emphasis on nosocomial transmission and infection prevention and control responses. DESIGN: A case series of patients admitted to WCH who had laboratory-confirmed cases of influenza between January 1 and July 31, 2009, with a comparison of patients with seasonal influenza and those with pandemic (H1N1) 2009 influenza; a review of the impact of infection prevention and control modifications on nosocomial transmission. PATIENTS AND SETTING: A total of 104 inpatients with influenza, 81 of whom had pandemic (H1N1) 2009 influenza, were reviewed at a large Canadian pediatric tertiary care center. RESULTS: There were no differences in risk factors, presentation, or outcome between patients with seasonal influenza and those with pandemic (H1N1) 2009 influenza. There were 8 nosocomial cases of pandemic (H1N1) 2009 influenza. Excluding patients with nosocomial cases, mean length of hospital stay was significantly shortened to 3.7 days for individuals who had pandemic (H1N1) 2009 influenza and who received empiric oseltamivir on admission to the hospital, compared with 12.0 days for patients for whom treatment was delayed (P = .02). Treatment with oseltamivir of all patients with suspected cases of influenza and prompt modifications to infection control practices, including playroom closures and enhanced education of visitors and staff, terminated nosocomial transmission. CONCLUSIONS: Infection with pandemic (H1N1) 2009 influenza virus resulted in a substantial number of hospitalizations of pediatric patients in Manitoba, including those with nosocomial cases, thereby stressing the capacity of WCH. Immediate therapy with oseltamivir on admission to the hospital resulted in a significantly reduced length of hospitalization. This, coupled with intensified infection prevention and control practices, halted nosocomial transmission. These strategies should be considered in future pandemic influenza or other respiratory viral outbreaks.

Second and first trimester estimation of risk for Down syndrome: implementation and performance in the SAFER study.

OBJECTIVES: Document patient choices and screening performance (false positive and detection rates) when three improved Down syndrome screening protocols were introduced coincidentally. METHOD: Second-trimester 'triple marker' screening was expanded by adding second-trimester dimeric inhibin-A (four-marker), with or without first-trimester pregnancy-associated plasma protein-A (five-marker). Nuchal translucency (NT) measurements were included when available from accredited sonographers (six-marker). For assigning risk, two sets of marker distribution parameters were evaluated. RESULTS: Over 3.5 years, 8571 women enrolled (median age 30.6 years). Uptake of the four-, five- and six-marker protocols was 18%, 46% and 36%, respectively. Of those selecting an integrated test (five or six markers), 9.7% did not provide the second trimester serum sample. False positive rates decreased with added markers (5.2%, 5.1% and 2.5%, respectively) and varied between the two parameter sets, while detection remained high. Overall, 21 of 23 cases were detected (91%, 95% CI 73-98%) at a 4.2% false positive rate (95% CI 3.3-5.1%). CONCLUSIONS: Integrated screening protocols were chosen 4.6 times more often than four-marker screening (82% vs. 18% uptake). Overall detection was higher and false positives lower, consistent with recent guidelines. Important performance factors include gestational dating method, risk cut-off, and the parameter set used to assign risk.