COVID-19 Vaccines: Emerging Strains & Booster Dose / Merck Discontinues Vaccine Efforts

By Kelly Young

Edited by David G. Fairchild, MD, MPH, and Jaye Elizabeth Hefner, MD

Moderna says that it will test whether an extra booster dose of its two-dose COVID-19 vaccine will provide additional protection against emerging strains of SARS-COV-2. Additionally, the company is beginning a phase 1 trial of a booster candidate, mRNA-1273.351, to test a booster with spike proteins specific to the B.1.351 variant, first detected in South Africa.

In an in vitro study posted on the preprint server bioRxiv, the company's FDA-authorized mRNA vaccine produced neutralizing antibodies against B.1.351 and B.1.1.7, the reportedly more contagious strain first detected in the United Kingdom. However, the neutralizing titers were six times lower with the B.1.351 variant than with earlier variants. Nonetheless, the company said that the levels of neutralizing antibodies were "above levels that are expected to be protective." Of note, President Joe Biden is expected to ban entry of non-U.S. citizens coming from South Africa to try to curb the spread of this variant in the United States, Reuters reports.

For the B.1.1.7 strain first found in the U.K., the Moderna vaccine prompted neutralizing titer levels similar to those seen for other variants. Last week, Pfizer-BioNTech reported that their mRNA vaccine also appeared effective against B.1.1.7.

In other vaccine news, Merck says it will halt study into two potential COVID-19 vaccines after early trials found they lacked efficacy compared with natural immunity and other available vaccines.