Safety and immunogenicity of SARS-CoV-2 variant mRNA vaccine boosters in healthy adults: an interim analysis

Nat Med. 2021 Nov;27(11):2025-2031. doi: 10.1038/s41591-021-01527-y. Epub 2021 Sep 15.


The emergence of SARS-CoV-2 variants of concern (VOCs) and variants of interest (VOIs) with decreased susceptibility to neutralization has generated interest in assessments of booster doses and variant-specific vaccines. Clinical trial participants who received a two-dose primary series of the COVID-19 vaccine mRNA-1273 approximately 6 months earlier entered an open-label phase 2a study ( NCT04405076 ) to evaluate the primary objectives of safety and immunogenicity of a single booster dose of mRNA-1273 or variant-modified mRNAs, including multivalent mRNA-1273.211. As the trial is currently ongoing, this exploratory interim analysis includes preliminary descriptive results only of four booster groups (n = 20 per group). Immediately before the booster dose, neutralizing antibodies against wild-type D614G virus had waned (P < 0.0001) relative to peak titers against wild-type D614G measured 1 month after the primary series, and neutralization titers against B.1.351 (Beta), P.1 (Gamma) and B.1.617.2 (Delta) VOCs were either low or undetectable. Both the mRNA-1273 booster and variant-modified boosters were safe and well-tolerated. All boosters, including mRNA-1273, numerically increased neutralization titers against the wild-type D614G virus compared to peak titers against wild-type D614G measured 1 month after the primary series; significant increases were observed for mRNA-1273 and mRNA-1273.211 (P < 0.0001). In addition, all boosters increased neutralization titers against key VOCs and VOIs, including B.1.351, P.1. and B.1.617.2, that were statistically equivalent to peak titers measured after the primary vaccine series against wild-type D614G virus, with superior titers against some VOIs. This trial is ongoing.

Publication types

  • Clinical Trial, Phase II
  • Multicenter Study
  • Randomized Controlled Trial
  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Adult
  • Aged
  • Antibodies, Neutralizing / blood
  • Antibodies, Viral / blood
  • COVID-19 / immunology
  • COVID-19 / prevention & control*
  • COVID-19 Vaccines / adverse effects
  • COVID-19 Vaccines / immunology*
  • Female
  • Healthy Volunteers
  • Humans
  • Immunization, Secondary* / adverse effects
  • Immunogenicity, Vaccine*
  • Male
  • Middle Aged
  • Preliminary Data
  • RNA, Messenger / adverse effects
  • RNA, Messenger / genetics
  • RNA, Messenger / immunology
  • SARS-CoV-2 / genetics
  • SARS-CoV-2 / immunology*
  • Treatment Outcome
  • United States
  • Vaccination / adverse effects


  • Antibodies, Neutralizing
  • Antibodies, Viral
  • COVID-19 Vaccines
  • RNA, Messenger

Supplementary concepts

  • SARS-CoV-2 variants

Associated data