The pandemic has caused the shipping industries around the world to go in lockdown for a while. About 80% of the world trade is carried out by commercial vessels and the lockdown can cause potential disruption and clogging up of the global supply chain.
The delay for crew exchanges makes the situation even worse. Seafarers have worked beyond their contracts for several months owing to the travel restrictions.
Singapore has done its bit by working with shipping companies, partners, seafarer unions...
https://www.marineinsight.com/shipping-news/3000-cases-of-crew-change-approved-in-singapore/