Table 9

Some critical unknowns in SARS-CoV-2 transmission

Critical unknownsConsiderations
Which transmission patterns will occur and will human-to-human transmission continue permanently?
  • Seasonal transmission in temperate climate?

  • Continuous tides, with ups and downs?

  • The experience from China and some other countries showed that ‘local elimination’ is possible but risk of reintroduction remains.

  • Increasingly unlikely that elimination everywhere is possible.

This will strongly depend on:
How strong will the acquired immunity after a first infection with SARS-CoV-2 be and how long will it last?
  • Evidence of acquired immunity against subsequent infections has been limited.

  • Measurable antibodies have been observed in most persons who have recovered from COVID-19,131 and research in animal models has suggested limited possibility of reinfection.121

  • It is still unclear as to how robust the immunity is and how long it will last.

  • Debate on use, practicality and ethics of ‘immunity passports’ for those recovered from COVID-19 has been ongoing.128 137

How stable is the virus (mutation) and do the different clades seen worldwide have any effect on the transmission potential/severity of the disease?
  • If the virus mutates quickly and different strains develop, then antibody-dependent enhancement might be an important risk, as in dengue with its four different strains. If so, then in subsequent waves progressively more severe cases could occur.

  • This has been reported for the Spanish influenza, where the second and third waves were characterised by a more severe disease pattern.36 84

What is the role of children in transmission?
  • Children have quasi-universally presented less severe disease. However, their susceptibility to infection remains unclear, with large heterogeneity reported between studies.138 139

  • Their role in transmission has remained unclear, but evidence points to a more modest role in transmission than adults.

How significant are asymptomatic carriers in transmission?
  • There have been several reports of asymptomatic transmission140 141 and estimates based on modelling.114

  • Increasing consensus that asymptomatic carriers play an important role in transmission.