Earlier this month, both Pfizer and Moderna voiced their expectation that demand for fourth doses is likely to develop. But in its interim statement on the subject, issued on Tuesday (May 17), the WHO says more data is needed before it can develop recommendations for future vaccination campaigns.
The WHO notes that data on fourth booster doses and beyond only exists for mRNA vaccines; and even then it is still sparse for this vaccine type.
Review of seven studies
The organization reviewed seven studies where a fourth COVID-19 vaccine dose has been given: six from Israel and one from Canada. All were conducted at a time when Omicron was the predominant circulating strain globally.
“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,” it notes.
“Data to support an additional dose for healthy younger populations are limited; preliminary data suggest that in younger people, the benefit is minimal.
“Moreover, follow-up time after the additional booster dose was limited, thereby precluding conclusions about duration of protection after this dose.
“Therefore, there is a lack of data to guide some important questions for making policy decisions. The limited available data suggest that for highest risk groups there is a benefit that supports the administration of an additional booster dose.”
Countries will need to balance the potential benefit of fourth doses with the financial costs, the organization continues.
“Administering an additional booster dose likely comes with considerable programmatic challenges in terms of vaccine delivery in many settings. The financial and opportunity cost of such programs must also be carefully weighed against the limited incremental benefit of an additional booster dose."
An evolving picture
Furthermore, there remains uncertainty over the evolution of the virus and new variants and how adjusted vaccines may protect against these. Meanwhile, the apparent seasonality of the virus means countries should take this into account when planning vaccination campaigns.
“In order to make sound policy decisions, data will need to be generated on the performance of current and variant-specific candidate COVID-19 vaccines, including the VE, immunogenicity and safety of an additional booster dose over time and by disease outcome and priority use groups," notes the WHO.
"More research is needed on the breadth, magnitude, and durability of humoral and cell-mediated immune responses to variants.
"Also needed is evidence to address other gaps in the evidence regarding the need for additional booster doses, which includes the duration of VE of inactivated, subunit and viral vectored vaccines over time and by disease outcome.
“Finally, an understanding of the vaccine correlates of protection and correlates of durability of protection in persons with and without previous COVID-19 infection would assist policy makers in creating sound programmatic decisions.”
