Moderna reported that during the phase 3 study of its vaccine candidate mRNA-1273, which enrolled 30,000 adult U.S. participants, just five of the 95 COVID-19 cases occurred among the vaccinated, while 90 infections were identified in the placebo group. This corresponds to an efficacy of 94.5%. None of the infected patients who received the vaccine developed severe COVID-19, while 11 (12%) of those who received the placebo did.
Similarly, the Pfizer-BioNTech vaccine candidate, BNT162b2, was 90% effective in preventing infection during the phase 3 clinical trial, which enrolled 43,538 participants, with 30% in U.S. and 42% abroad
Vaccines train the immune system to recognize the disease-causing part of a virus. Vaccines traditionally contain either weakened viruses or purified signature proteins of the virus.
But an mRNA vaccine is different, because rather than having the viral protein injected, a person receives genetic material – mRNA – that encodes the viral protein. When these genetic instructions are injected into the upper arm, the muscle cells translate them to make the viral protein directly in the body.
This approach mimics what the SARS-CoV-2 does in nature – but the vaccine mRNA codes only for the critical fragment of the viral protein. This gives the immune system a preview of what the real virus looks like without causing disease. This preview gives the immune system time to design powerful antibodies that can neutralize the real virus if the individual is ever infected.
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