Why there are so many COVID-19 vaccines in development :

• There are many different COVID-19 vaccines in development because it is not yet known which ones will be effective and safe
• Based on experience, roughly 7% of vaccines in preclinical studies succed. Candidates that reach clinical trials have about a 20% chance of succeeding
• Different vaccine types may be needed for different population groups
• For example, some vaccines may work in older persons and some may not, as the immune system weakens with older age