Origin of SARS-CoV-2 | |
Transmission | Mainly through respiratory droplets from infected persons105; by hands, after contamination at nose, mouth or eyes; also through air on exposure to sneezing or coughing from an infected person at close distance. Through aerosols, while singing/talking loudly in congregations, groups, parties, karaoke, and so on, especially in poorly ventilated spaces.18 106 Through fomites.107 Possibly via faecal–oral route108 109; detection in sewage.110–112 Related to peak in upper respiratory tract viral load prior to symptom onset in presymptomatic (paucisymptomatic) persons.106 113 114 Transmission dynamics in asymptomatic persons not fully elucidated although viral shedding occurs.115
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Influence of climate and/or air pollution on transmission | Influence of climate on the capacity of the virus to survive outside human body (in air, in droplets, on surfaces, etc.) and to spread has been speculative. May spread more readily in milder/colder climate116 117; although variability of the reproductive number could not be explained by temperature or humidity.118 Existing levels of air pollution may play a role; air pollutants, such as particulate matter, nitrogen dioxide and carbon monoxide, are likely a factor facilitating longevity of virus particles.119 Elevated exposure to common particulate matter can alter host immunity to respiratory viral infections.119
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Immunity—protective antibodies | IgM and IgA antibody response 5–10 days after onset of symptoms, does not depend on clinical severity, correlates with virus neutralisation; IgG is observed ~14 days after onset of symptoms,93 120 may or may not correspond to protective immunity. Whether antibody response is long lasting has remained unclear. Rechallenge in rhesus macaques showed immunity post primary infection.121 How protective immunity after first infection is against subsequent infection with an identical or mutated strain has been uncertain. Incidental reports showed recovered persons positive by real-time PCR,122 123 later attributed to testing errors.
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Seroprevalence to SARS-CoV-2 | Reported estimates for seroprevalence range between 0.4% and 59.3%124; differences in timing of the serosurvey, the use of assay kits with varying sensitivity/specificity, and different methods for detection may contribute to this large variation.125 Seemingly high seroprevalence may be due to cross-reactive epitopes between SARS-CoV-2 and other HCoVs.126 127 Whether seroprevalence implies immune protection is unclear, yet, some countries have considered use of ‘immunity passports’.128 For herd immunity to be effectively achieved, an estimated seroprevalence of 60% of the population will be required.37 Other studies estimate between 5.66% and 85% seroprevalence in different countries.129–131
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Rate of variability/mutation in SARS-CoV-2 | Mutation rate: ~10−3 substitutions per year per site132 133; 13 mutation sites have been identified as of May 2020, suggesting selective mutations.134 The low mutation rate suggests that a vaccine would be a single vaccine rather than a new vaccine every year like the influenza vaccine.135 Ten different circulating clades (nextstrain.org; www.gisaid.org),136 as of May 2020.
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