People 60 years of age and older who were initially vaccinated with two Pfizer-BioNTech COVID-19 vaccine doses were better protected against the omicron coronavirus variant after being boosted with a Moderna vaccine rather than a different dose of the Pfizer vaccine. BioNTech vaccine.
Those results are according to interim data from a small but randomized controlled clinical trial in Singapore and published this week in the journal Clinical Infectious Diseases.
The study — which involved 98 healthy adults — cannot determine whether the Moderna booster is simply superior to a Pfizer-BioNTech booster for older adults or whether a mix-and-match booster strategy is inherently better. It also focused solely on antibody levels, which may or may not translate into significant differences in infection rates and other clinical differences. It also tracked people for just 28 days after a booster, so it’s unclear if the Moderna booster’s lead holds up over time.
Still, the authors of the study, led by Barnaby Young of Singapore’s National Center for Infectious Diseases, report that the beneficial effect seen by switching from Pfizer-BioNTech to Moderna was significant enough that they didn’t expect to see more participants. would disappear. It also follows other studies that have suggested that mix-and-match boosting — also called heterologous boosting — may generate slightly different antibodies and reduce the incidence of SARS-CoV-2 infections in people 60 and older.
For the new study, Young and colleagues looked at antibody levels in adults of all ages who had received two Pfizer-BioNTech COVID-19 vaccine doses between six and nine months before receiving a booster dose. The researchers excluded people from the trial if they had a compromised immune system or had evidence of previous SARS-CoV-2 infections (the presence of anti-N antibodies).
Of the 98 participants, 50 received another Pfizer-BioNTech vaccine dose for their booster (homologous booster), while the remaining 48 received a Moderna booster (heterologous booster). The authors looked at their resulting antibody responses on the day of their booster, seven days later, and 28 days later. Specifically, they compared the total levels of antibodies that targeted a key component of the SARS-CoV-2 spike protein, the receptor-binding domain. They also looked at levels of neutralizing antibodies against a range of specific SARS-CoV-2 variants, from the ancestral strain to alpha, beta, delta and omicron.
Slightly bigger boost
Overall, the heterologous-enhanced group had slightly higher total antibody levels than the homologous group — about 40 percent higher on day seven and 30 percent higher on day 28, although the confidence intervals overlapped. But when the authors broke down the groups by age, they found that the benefit came entirely from differences in the age group 60 and older. Antibody levels were similar among younger participants regardless of booster type.
Among the over-60s, there were 24 homologous-enhanced participants and 23 heterologous-enhanced participants. Seven days after the booster, the heterologous-boosted participants had two times higher antibody levels than the homologous group and 60 percent higher levels after 28 days.
Older participants with a heterologous boost also had higher levels of neutralizing antibodies against all SARS-CoV-2 variants tested — with the greatest difference seen against omicron, which is notorious for antagonizing vaccine-derived immune responses. After seven days, the level of neutralizing antibody inhibition was 89 percent in the heterologous-enhanced group compared to 64 percent in the homologous-enhanced group. After 28 days, the range was 84 percent in the heterologous-enhanced group to 73 percent in the homologous-enhanced group.
Overall, Young and co-authors concluded: “Especially for the vulnerable older age group, a heterologous booster COVID-19 vaccine regimen induces a higher anti-spike antibody titer and a stronger neutralizing antibody response against the highly infectious Omicron variant (~20 percent higher neutralization) than a homologous booster regimen.”
The trial is still ongoing, so the authors will continue to add participants and data. They plan to reassess the antibody response in all participants six months and 12 months after the booster. They will add to the study people who were initially given Moderna vaccines to see if switching to the Pfizer-BioNTech vaccine for the booster provides a similar benefit.
