A new phase II study from Moderna shows that half-doses (50 μg) appear to be as good as full doses (100 ug) at generating correlates of protection such as neutralizing antibodies.
In this randomized, controlled phase 2 trial, the SARS-CoV-2 vaccine candidate mRNA-1273, administered as a two-dose vaccination regimen at 50 and 100 μg, exhibited robust immune responses and an acceptable safety profile in healthy adults aged 18 years and older. Local and systemic adverse reactions were mostly mild-to-moderate in severity, were ≤4 days of median duration and were less commonly reported in older compared with younger adults. Anti-SARS-CoV-2 spike binding and neutralizing antibodies were induced by both doses of mRNA-1273 within 28 days after the first vaccination, and rose substantially to peak titers by 14 days after the second vaccination, exceeding levels of convalescent sera from COVID-19 patients. The antibodies remained elevated through the last time point assessed at 57 days. Neutralizing responses met criteria for seroconversion within 28 days after the first vaccination in the majority of participants, with rates of 100% observed at 14 and 28 days after the second vaccination. While no formal statistical testing was done, binding and neutralizing antibody responses were generally comparable in participants who received the 100 μg mRNA-1273 and the 50 μg dose at all time points and across both age groups. Overall, the results of this randomized, placebo-controlled trial extend previous immunogenicity and safety results for mRNA-1273 in the phase 1 study in an expanded cohort including participants older than 55 years of age [16, 19].
[These data] confirm that a robust immune response is generated at both 50 and 100 ug dose levels.
As I wrote earlier, halving the dose is equivalent to instantly doubling the output of every Moderna factory.
See my piece in the Washington Post on getting to V-day sooner for an overview of dose stretching strategies.
Addendum: France says one dose is sufficient for previously COVID infected.