Second line drugs are administered when first line drugs are ineffective in treating a disease or when there are side effects. First line drugs are those drugs which are administered to treat diseases, and they are chosen due to their high clinical effectiveness and low side-effects. You can read about mRNA Vaccine: Overview, Mechanism and Advantage in the given link.
- Topic-Wise GS 3 Questions for UPSC Mains
- Science & Technology Notes For UPSC – General Science Notes