 "vaccination")
The WHO has said that there is benefit of administering an additional booster dose of the mRNA COVID-19 vaccine for highest risk groups, health workers, those over 60 years of age or with immunocompromising conditions.
As a general principle, an interval of 4-6 months since completion of the primary series of the vaccine could be considered for a first booster, especially in the context of Omicron, said the World Health Organisation, with the support of the Strategic Advisory Group of Experts (SAGE) on immunisation and its COVID-19 Vaccines Working Group.
The WHO said that in considering additional booster doses, the two main scenarios to assess are the use of additional booster doses in those who are not able to mount and sustain adequate immune responses and considerations for additional booster doses to be administered in order to protect high risk populations and health workers to maintain the health system during periodic waves of disease surges.
It said that available data for WHO EUL COVID-19 vaccine products suggest that vaccine effectiveness and immunogenicity are lower in immunocompromised persons (ICPs), compared to persons without immunocompromising conditions.
An additional dose included in an extended primary series enhances immune responses in some ICPs. Given the significant risk of severe COVID-19 for ICPs, if infected, WHO has already issued a recommendation for an extended primary series (i.e. third dose) as well as a booster dose (i.e. fourth dose) for ICPs, for all COVID-19 vaccines, it said.
The WHO noted that additional booster doses beyond the first booster dose are currently being offered by some countries, adding that data on additional booster doses as of May 2022 only exists for the mRNA vaccines, and not for other vaccine platforms.
The WHO cited seven studies, conducted during a time when Omicron was the predominant circulating strain globally, that evaluated the relative effectiveness of a fourth dose four months after a third dose of mRNA vaccine compared to those who received three doses.
Taken together, these studies show some short-term benefit of an additional booster dose of mRNA vaccine in health workers, those over 60 years of age or with immunocompromising conditions, the WHO said.
It said that data to support an additional dose for healthy younger populations are limited; preliminary data suggest that in younger people, the benefit is minimal.
The limited available data suggest that for highest risk groups, there is a benefit that supports the administration of an additional booster dose, the WHO said, adding that in those most at risk for severe disease or death (i.e. adults above the age of 60 years, or those who are not able to mount a full immune response), the additional benefit of an additional booster dose of mRNA vaccine might be warranted.
It said while seasonality is not yet fully established for SARS-COV-2, evidence from the past two years support the notion of more substantial transmission during the winter season.
Therefore, for countries with either a Northern or Southern Hemisphere winter season, plans for catch-up to improve primary series coverage and boosting for those at highest risk, campaigns should take seasonality into account, the WHO added.
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