Some nations are already looking to a fourth vaccine dose to help contend with a huge omicron-driven spike in COVID-19 cases, but early signs suggest repeat vaccination may be a hard sell as beleaguered populations enter their third pandemic year.

Uptake of the first round of boosters in recent months — a third shot for most — has lagged initial vaccinations.

Disease experts say that rapidly shifting public health messaging in the face of a quickly mutating virus has bred confusion and mistrust over the benefit of boosters.