The World Health Organization (WHO), with the support of the Technical Advisory Group on COVID-19 Vaccine Composition (TAG-CO-VAC), continues to review and assess the public health implications of emerging SARS-CoV-2 Variants of Concern (VOC) on the performance of COVID-19 vaccines. Since the publication of the interim statement on COVID-19 vaccines on 11 January 2022, Omicron has become the dominant VOC globally, rapidly replacing other circulating variants. This statement highlights the global epidemiological situation, challenges of updating vaccine composition and provides the current position of the TAG-CO-VAC.
The current global epidemiological situation is characterized by rapid and relatively synchronous dominance of Omicron variant in all six WHO regions. While global cases are declining, there are reduced testing resources and capacities in some areas and the epidemiological situation remains heterogeneous, with a number of regions and countries reporting increases in new weekly cases, while others are now reporting declines.
Omicron is comprised of several genetically related sublineages, including BA.1, BA.2 and BA.3, each of which is being monitored by WHO and partners. At a global level, BA.1 has been the predominant Omicron lineage, however, the proportion of reported sequences designated as BA.2 has been increasing relative to BA.1 in recent weeks, and is the predominant Omicron lineage in several countries. BA.1 and BA.2 have some genetic differences, which may make them antigenically distinct.
Reinfection with BA.2 following infection with BA.1 has been documented, however, initial data from population-level studies suggest that infection with BA.1 provides substantial protection against reinfection with BA.2, at least for the limited period for which data are available.
The public health goal of COVID-19 vaccination prioritizes protection against severe disease and death. Current vaccines appear to confer high levels of protection against severe disease outcomes associated with Omicron infection. The TAG-CO-VAC therefore strongly supports urgent and broad access to current COVID-19 vaccines for primary series and booster doses, particularly for groups at risk of developing severe disease. The near- and medium-term supply of the available vaccines has increased substantially, however, vaccine equity remains an important challenge and all efforts to address such inequities are strongly encouraged.
Current vaccines are based on the virus that circulated early in the pandemic (ancestral virus e.g., GISAID: hCoV-19/Wuhan/WIV04/2019). Since then, there has been continuous and substantial virus evolution and it is likely that this evolution will continue, resulting in the emergence of new variants. The composition of current COVID-19 vaccines may therefore need to be updated. Any update to current COVID-19 vaccine composition would aim to, at a minimum, retain protection against severe disease and death, while ensuring the breadth of the immune response against circulating and emerging variants, which may be antigenically distinct.
The TAG-CO-VAC considered a number of issues, all of which are important in any decision on COVID-19 vaccine composition:
- There are heterogeneous levels of population immunity between countries due to different waves of VOCs and different types, levels and timing of vaccination, but robust data on the global immunologic landscape are limited. The performance of any updated vaccine(s) may vary depending on the nature and magnitude of previously acquired immunity.
- When updated vaccines become available, a substantial proportion of the global population will have been exposed to SARS-CoV-2, either as a result of vaccination and/or prior infection. As above, the performance of any updated vaccine(s) may vary depending on the nature and magnitude of previously acquired immunity.
- There are also considerable uncertainties as to how the virus will continue to evolve and the antigenic characteristics of future variants. Given widespread transmission of Omicron globally, the possibility of its continued evolution is high and a new variant may emerge before an updated vaccine can be produced and delivered at scale.
WHO is tracking lineages under the ‘umbrella’ of Omicron, including BA.1 and BA.2. Though data are emerging, additional antigenic and virologic characterization of these lineages is needed both independently and in comparison, to the other lineages.
While the body of evidence on the immune response to Omicron following infection is rapidly growing, data on breadth, magnitude, and durability of humoral and cell-mediated immune responses to variants from variant-specific candidate vaccines using different vaccine platforms remain limited.
In addition to the current COVID-19 vaccines, there are many other COVID-19 vaccines in various stages of clinical and preclinical development. Any decision from the TAG-CO-VAC on COVID-19 vaccine composition would apply primarily to current COVID-19 vaccines.
