evidence: How it works: Similar to the Pfizer vaccine, this is an mRNA vaccine that sends host cells in the body instructions for making a spike protein that will train the immune system to recognize it. The immune system will then attack the spike protein the next time it sees one (attached to the actual SARS CoV-2 virus). [...] Moderna’s initial Phase 3 clinical data in December 2020 was similar to Pfizer-BioNTech’s—both vaccines showed about 95% efficacy for prevention of COVID. Later data on real-world effectiveness for adults showed that the protection from the mRNA two-dose primary series wanes over time, but booster doses brought the immune system back to robust levels. Additional information is available on the FDA’s Moderna 2024-2025 vaccine fact sheet.