| 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.
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| 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
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| 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
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| 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.
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| How significant are asymptomatic carriers in transmission? | |