The WHO interim recommendations on the use of the Janssen Ad26.COV2.S (COVID-19) vaccine (updated on 9 December) were developed on the basis of advice issued by the Strategic Advisory Group of Experts (SAGE) on Immunization and the evidence summary is included in their documentation.
Summary of the recommendations:
Countries can now consider using either a one- or two-course of the Janssen vaccine. The one-dose schedule is an EUL authorized regimen.
In some circumstances, using one dose may have advantages. Many countries face severe vaccine supply constraints, combined with a high disease burden. A single dose of the vaccine is efficacious and makes it possible to increase vaccine coverage rapidly, which in turn will reduce the burden on health care systems by preventing severe disease outcomes. A single dose may also be a preferred option for vaccinating hard-to-reach populations or populations living in conflict or insecure settings.
A second dose may be appropriate as vaccine supplies and/or accessibility increases. Countries should consider offering a second dose, beginning with the highest priority populations (e.g. healthcare workers, older people, people with comorbidities) as indicated in the WHO Prioritization Roadmap. The administration of the second dose will result in increased protection against symptomatic infection, and against severe disease.
A heterologous vaccine (e.g., a COVID-19 vaccine from another vaccine platform that has received EUL) can also be considered for the second dose.
Countries can also consider a longer interval between doses. A second dose 2 months after the initial dose substantially increases efficacy, especially against symptomatic infections, including when caused by SARS-CoV-2 variants of concern.
An even longer interval between the two doses with Ad26.COV2.S (6 months rather than 2 months) has been shown to result in a larger increase in immune responses in adults. Countries could therefore consider an interval of up to 6 months based on their epidemiological situation, and needs of sub-populations.