Table 6

Knowns, uncertainties and unknowns about severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), as of May 2020

FactorAvailable information
Origin of
  • Most probably from bats via intermediate animal hosts to index case.47 104

  • All subsequent cases resulted from human-to-human 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

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

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.

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

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 (;,136 as of May 2020.

  • HCoV, human coronaviruses.