New variants of concerns
New variants of SARS-CoV-2 emerged sequentially, becoming the
predominant strains during the pandemic. These variants have distinct
ACE2 binding affinity, virulence, transmissibility and host immune
responses88-100 (Figure 1). Currently, the most
pronounced risk factor for SARS-CoV-2 infection is the emergence of new
variants or subvariants that are resistant to neutralizing antibodies
and with higher transmissibility101. The subvariants
BA.4 and BA.5, most likely stem from Omicron lineage
BA.2101, were firstly detected in South
Africa99 and are now spreading in Europe and the
United States102. BA.4 and BA.5 have become the
dominant VOC in many European countries. The hospitalization and death
rate of BA.4 and BA.5 were significantly lower compared to previous
waves of infection in South Africa99, which may be due
to the high population immunity. However, in Portugal, the
hospitalization and mortality caused by BA.4/5 were similar to that in
the first wave of Omicron infection, which may be due to the higher
proportion of elderly individuals in this country101.
BA.4 and BA.5 carry additional mutations in the spike proteins assisting
the immune escape induced by 3-dose vaccinations and by post-vaccination
infection of BA.1103,104. The Omicron variant is
continuously evolving escape antibody neutralization resulting in
breakthrough infection of SARS-CoV-2 in both vaccinated and in
previously infected individuals105. The biological and
clinical characteristics of rhinovirus, influenza A, SARS-CoV-2 variants
delta, Omicron BA.1 and BA.5 are compared in Table 1.