Measurement of anti-neuraminidase antibodies in addition to anti- hemagglutinin antibodies may be important in capturing the true influenza infection rates.
We conducted a multicentre cross sectional observational study of laboratory, public health and hospitalisation data for PCR-confirmed COVID-19 cases within the New Zealand Northern Region, between 12 February and 8 June 2020. The aim of this study was to describe population level SARS-CoV-2 upper respiratory tract (URT) viral load dynamics by stratifying positivity rates and polymerase chain reaction (PCR) cycle threshold (Ct) values of URT samples from COVID-19 cases by days since symptom onset, and to explore utility of Ct values in determining length of time post-infection and thus potential infectivity. Of 123,124 samples tested for SARS-CoV-2 by PCR, 579 samples (407 positive and 172 negative) from 368 symptomatic non-hospitalised individuals with PCR-confirmed infection were included. Sample positivity rate was 61.5% (8/13) for pre-symptomatic samples, rising to 93.2% (317/340) for samples collected during the purported symptomatic infectious period (days 0-10 post-symptom onset), and dropping to 36.3% (82/226) for postinfectious period samples (day 11 onwards). URT viral load peaked shortly after symptom onset, with median Ct values ranging 20.00-29.99 until 15 days post-symptom onset, and >30.00 after this time. Of samples with a Ct value of <20.00, 96.1% were collected during the symptomatic infectious period. However, of samples with a Ct value 30.00 and 35.00, 46.9% and 18.5%, respectively, were also collected during the symptomatic infectious period. The findings of this study indicate that at or soon after symptom onset represents the optimum time to test for SARS-CoV-2 in the URT, with median Ct values suggesting the useful testing window extends until around 15 days post-symptom onset. In asymptomatic individuals or those with unknown dates of symptom onset, Ct values <20.00 imply recent onset/potential infectivity, but Ct values 30.00 or 35.00 do not exclude recent onset/potential infectivity. Individual sample Ct values should not be used as an absolute marker of length of time post-infection or to exclude infectivity where date of symptom onset is unavailable.
Highlights d Post-infection seroconversion is associated with severity of influenza virus infection d Seroconverters have early proliferation and activation of CD4 + T cells d CD8 + T cells are unaffected d CD14 hi CD16 + monocytes in the blood and nasal mucosa is associated with antibody response
Case report: A previously well 34-year-old man with mild COVID-19 symptoms tested positive by SARS-CoV-2 RT-PCR on 14/08/2020 (Ct 17.6). He was epidemiologically and genomically linked to the Auckland August 2020 cluster. He represented with acute breathlessness 150 days later on 11/01/ 2021, and tested positive on two different SARS-CoV-2 RT-PCR assays (Ct 36.9/38.4). At this time, excluding two discrete community clusters, COVID-19 transmission had been eliminated in New Zealand since May 2020. The patient denied infective symptoms and could not be linked to any imported COVID-19 cases or high risk exposures. His breathlessness was due to bronchial obstruction from sarcoidosis mediastinal lymphadenopathy. Genome sequencing of the 11/01/ 2021 result failed to yield analysable sequence, but re-infection was unlikely, and with an alternative diagnosis for his breathlessness the result was attributed to prolonged upper respiratory tract viral shedding. He was discharged without further isolation. He had tested negative twice in the interim since his initial positive result. In individuals with previous COVID-19, sporadic positive RT-PCR results may be obtained many months after initial infection, even with multiple negative results in the interim. This represents one of the longest shedding durations reported, and has important implications for interpretation of RT-PCR results in previously infected individuals.
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