[Evidence summary]: Is there evidence of mRNA vaccine effectiveness with an extended dosing interval? If so, what is the interval? [V1.0]

Main Points 1. The WHO and CDC recommend that in exceptional epidemiological circumstances, countries may consider extending the dosing interval to a maximum of 42 days . Currently, only limited data are available on the efficacy of mRNA COVID 19 vaccines administered beyond the 42 day window. 2. Modelling studies suggest that initially vaccinating a greater number of people with a single dose will prevent more deaths and hospitalizations than vaccinating a smaller number of people w ith 2 doses ; and that extending the dosing interval shows progressive benefit to population immunity 3. Data suggest that the second dose should not be delayed in t hose >65 years of age or in immunosuppressed individuals such as cancer patients or transplant recipients due to reduced vaccine immunogenicity after the first dose.