The policy on second booster doses needs to take into account each country’s demographics, current and potential burden of severe COVID-19 disease and hospitalizations, availability and access to vaccines, opportunity costs, coverage rates with primary series and community acceptance of second boosters, notes the WHO.
To reduce the risk of severe disease, deaths and disruption to health services, WHO recommends that countries consider a second booster dose for the following population groups:
- all older persons (age specific cut-off should be defined by countries based on local COVID-19 epidemiology);
- all persons with moderately and severely immunocompromising conditions
- adults with comorbidities that put them at higher risk of severe disease;
- pregnant women; and
- health workers.
Vaccine effectiveness and variant-specific vaccines
In the context of the Omicron variant, waning of vaccine effectiveness (VE) is considered by the WHO to be relatively minor against severe disease and significant and rapid against symptomatic illness.
“A recent systematic review and meta-regression assessing VE over time since vaccination examined data from 3 December 2021 to 21 April 2022 during an Omicron-dominant period (BA.1). Using random-effects meta-regression, the mean change in effectiveness was estimated 1–4 months after the first booster vaccination (i.e. third dose)," notes the SAGE committee, in its latest set of recommendations, issued on August 18 following the meeting of the Strategic Advisory Group of Experts on Immunization (SAGE) on August 11.
"The booster dose increased VE against all outcomes initially. Over time the decrease in effectiveness against severe disease was found to be 5% (95% CI: 2–9) 1–4 months after booster vaccination, and 8% (95% CI: 4–14) when projected to 6 months after booster vaccination.
"In contrast, against symptomatic disease, there was a decrease in VE over time of 24% (95% CI: 20–29) 1–4 months after the booster vaccination, and of 29% (95% CI: 18–41) when projected to 6 months.
“The rationale for a second booster is to restore and possibly enhance protection. In some countries, second booster doses are currently being offered (i.e. fourth doses to older adults and fifth doses to immunocompromised persons).
"Because these doses have been administered relatively recently, data are limited on the additional protection they confer in terms of the duration of VE, and may differ by vaccine platform.”
Moderna’s Spikevax Bivalent Original/Omicron became the first next-generation Omicron-containing bivalent vaccine to be authorized on August 15, getting the green light from regulators in the UK. Others vaccine manufacturers are also in the late-stages of developing Omicron-specific vaccines, such as Pfizer and BioNTech, who have submitted applications for authorization in the US and EU for their bivalent Omicron shot.
However, current vaccines based on the index virus appear to maintain high efficacy against severe COVID-19 and therefore the roll-out of second booster doses to appropriate groups should not be delayed while waiting for variant-containing vaccines to be distributed, says the WHO.
Long-term vaccination policies
The WHO suggests COVID-19 vaccine could be administered alongside flu jabs on a seasonal basis.
“For longer-term considerations, there are significant uncertainties related to the evolution of the virus, the characteristics of future variants, and the trajectory of the epidemic given increasing vaccine- and infection-induced immunity globally.
"It is likely that additional doses may be needed within 4–12 months after the second booster, especially in persons most vulnerable to severe disease and death.
“Despite the potential benefit of variant-containing vaccines, current vaccines, based on the index virus, appear to maintain high VE against severe disease in the context of the Omicron variant and its sub-lineages. The administration of a second booster dose, as recommended by this guidance, should not be delayed in anticipation of future variant-containing vaccines.
“Once variant-containing vaccines are available, WHO will assess their performance and update guidance, as needed.”
However, the WHO notes that pan-SARS-CoV-2 or pan-sarbecovirus vaccines, as well as vaccines with greater impact on virus transmission (such as nasal vaccines), are urgently needed: with the time frame for candidates in development remaining uncertain.