People who received the one-dose Johnson & Johnson COVID-19 vaccine may need a booster shot to fend off some of the worrisome coronavirus variants now spreading worldwide, a study suggests.

A “significant fraction” of blood samples from recipients of the J&J shot had low neutralizing antibody levels against the Delta, Delta Plus, Beta and Lambda variants, according to a report posted on Wednesday on the medical website bioRxiv ahead of peer review.

Neutralizing antibody levels were so low, in fact, that the vaccine would be unlikely to offer even 50% protection against infection with those viruses, researchers from the New York University Grossman School of Medicine said.

In the United States, 50% efficacy is the minimum for FDA emergency-use approval, which the J&J vaccine has received. Based on studies with other vaccines, the researchers believe neutralizing antibody levels in recipients of the J&J vaccine could be improved either with a second dose of the same vaccine or with a dose of an mRNA vaccine from Pfizer/BioNTech or Moderna.