In some studies, the effectiveness of inactivated vaccines (Sinovac-CoronaVac and Sinopharm-BBIBP) against severe disease and death was lower in older persons than in younger adults. Furthermore, immune responses generated following a complete vaccination series were lower in persons above 60 years of age in whom seropositivity declined more rapidly than in younger persons[^1] [^2].
WHO therefore recommends administration of an additional dose of Sinovac-CoronaVac and Sinopharm-BBIBP vaccines to older adults 3-6 months after the second dose.
[^1]: WHO, Geneva, Switzerland. Interim recommendations for use of inactivated COVID-19 vaccine, CoronaVac, developed by Sinovac. Updated 21 October 2021. Available at <https://www.who.int/publications/i/item/WHO-2019-nCoV-vaccines-SAGE_recommendation-Sinovac-CoronaVac-2021.1>. Accessed on 2 November 2021.
[^2]: WHO, Geneva, Switzerland. Interim recommendations for use of inactivated COVID-19 vaccine BIBP developed by China National Biotec Group (CNBG), Sinopharm. Available at <https://www.who.int/publications/i/item/WHO-2019-nCoV-vaccines-SAGE-recommendation-bbv152-covaxin>. Accessed on 2 November 2021.