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As of July 2021, over 3 billion doses of a COVID-19 vaccines have been administered globally, and there are now 19 COVID-19 vaccines approved for use in at least one country. Several of these have been shown to be highly effective both in clinical trials and real-world observational studies, some of which have included special populations of interest. A small number of countries have approved a COVID-19 vaccine for use in adolescents or children. These are laudable achievements, but the global vaccination effort has been challenged by inequitable distribution of vaccines predominantly to high income countries, with only 0.9% of people in low-income countries having received at least one dose of a COVID-19 vaccine. Addressing this inequity is of critical importance and will result in better control of SARS-CoV-2 globally. Other challenges include: the reduced protection from COVID-19 vaccines against some strains of SARS-CoV-2, necessitating the development of variant specific vaccines; and uncertainties around the duration of protection from vaccine-induced immunity.

There is a strong consensus globally that a COVID-19 vaccine is likely the most effective approach to sustainably controlling the COVID-19 pandemic. An unprecedented research effort and global coordination has resulted in a rapid development of vaccine candidates and initiation of trials. Here, we review vaccine types, and progress with 10 vaccine candidates against SARS-CoV-2 - the virus that causes COVID-19 - currently undergoing early phase human trials. We also consider the many challenges of developing and deploying a new vaccine on a global scale, and recommend caution with respect to our expectations of the timeline that may be ahead.