Washington: The road to eliminating COVID-19 is long and paved with uncertainty. Many countries are counting on vaccines to build sufficient immunity in their populations so that SARS-CoV-2 isn’t able to find susceptible people to infect, causing transmission of the coronavirus to slow and eventually stop. But even with the rollout of highly effective vaccines, immunisation coverage may not reach that level - the so-called herd immunity threshold - anytime soon. For one thing, it’s not known what level of immunity is required. There’s also the threat of emerging coronavirus variants that may weaken the effectiveness of immunisations.