World Health Organization (WHO) chief scientist Soumya Swaminathan said Monday, January 11 that herd immunity to the coronavirus would not be achieved in 2021, despite the growing availability of vaccines, Deutsche Welle reports.

Mitigating factors to herd immunity include limited access to vaccines in developing countries, skepticism about vaccination and the potential for virus mutations, according to health experts.

A growing number of countries around the world — including the United States, the United Kingdom, Singapore, and Germany and other EU countries — are in the first stages of mass-vaccination campaigns.

Herd immunity occurs when enough people in a population have immunity to an infection so that it prevents the disease from spreading.

In the United States, which currently has the world's highest daily case numbers, officials said Monday that over 25.4 million vaccine doses have so far been distributed, with nearly 9 million doses having been administered. In Germany, more than 600,000 people have so far been vaccinated against Covid-19, according the Robert Koch Institute for public health.