The Comirnaty (Pfizer/BioNTech) and Moderna COVID-19 vaccines.
Enlarge / The Comirnaty (Pfizer/BioNTech) and Moderna COVID-19 vaccines.

People ages 60 and older who were initially injected with two Pfizer-BioNTech COVID-19 vaccine doses were better protected from the omicron coronaviruses variant after being boosted with a Moderna vaccine.

Interim data from a small but randomized controlled clinical trial in Singapore was published this week in the journal Clinical Infectious Diseases.

The study can not determine if the Moderna booster is superior to the Pfizer-BioNTech booster for older adults. It only focused on the levels of the antibodies, which may or may not translate to differences in the rates of infections. It is unclear if the Moderna booster will hold up over time, as it only followed people for 28 days after a booster.

The beneficial effect seen by changing from Pfizer-BioNTech to Moderna was significant enough that they don't expect it to disappear with more participants. It also follows other studies that suggest that mix-and-match boosting can reduce the incidence of the disease in people over the age of 60.

Young and colleagues looked at the antibody levels of adults who had received two Pfizer-BioNTech COVID-19 vaccine doses between six and nine months before receiving a booster dose. The people who were excluded from the trial were those who had compromised immune systems or had evidence of prior infections of the coronaviruses.


50 of the 98 participants got another Pfizer-BioNTech vaccine dose for their booster, while 48 got a Moderna booster. The authors looked at their responses on the day of their booster, seven days later and 28 days later. They compared the levels of antibodies that targeted a key part of the SARS-CoV-2 spike protein. They looked at levels of neutralizing antibodies against a range of specific SARS-CoV-2 variants, from the ancestral strain to alpha, delta, and omicron.

Slightly bigger boost

The group with the higher total antibody levels was about 40 percent higher on day seven and 30 percent higher on day 28. The authors found that the benefit was due to differences in the 60-and-up group. Younger participants had the same levels of Antibody as older participants.

There were 24 homologous-boosted participants and 23 Heterologous-boosted participants among those 60 and older. At seven days after the booster, participants in the Heterologous-boosted group had two-fold higher antibody levels than those in the homologous group.

The largest difference seen against omicron was the one that had higher levels of neutralizing antibodies against all of the SARS-CoV-2 variant. At seven days, the level of neutralizing antibody inhibition was 89 percent in the Heterologous-boosted group compared with 64 percent in the homologous-boosted group. The spread was 84 percent in the Heterologous-boosted group to 73 percent in the homologous-boosted group at 28 days.

For the vulnerable older age group in particular, a booster regimen of COVID-19 vaccine has been shown to induce a higher anti-spike titer and a stronger neutralizing response against the Omicron variant.

The trial is still going on, so the authors will keep adding participants and data. They plan to reexamine the responses of participants six months and a year after the booster. People who initially received Moderna vaccines will be added to the study to see if the Pfizer-BioNTech vaccine offers the same benefit.