New York, Jan 27 : A booster dose of any Covid vaccine is safe and prompts an immune response among people who had previously received a full regimen of any authorised Covid-19 vaccines, according to a preliminary clinical trial conducted by the US National Institutes of Health. The findings, reported in The New England Journal of Medicine, showed that combinations of primary and booster vaccine resulted in increased neutralising antibody levels (ranging from 4.2- to 76-fold higher levels than those detected prior to boost.) Likewise, all primary-boost combinations increased binding antibody levels 4.6- to 56-fold. The new report describes findings from 458 adults who had been fully vaccinated with any of three authorised Covid vaccines in the US -- Pfizer, Moderna and Johnson and Johnson -- at least 12 weeks prior to enrollment and who had no reported history of SARS-CoV-2 infection. At enrollment, a single booster dose was administered to each participant: 150 received Janssen/Johnson & Johnson's vaccine; 154 received Moderna; and 154 received Pfizer-BioNTech shot. Depending on which primary vaccine regimen a participant had received, the booster vaccine was either different (mixed, or heterologous) than or the same (matched, or homologous) as the original vaccine. The trial participants kept diaries of any side effects. More than half of participants reported headache, pain at the injection site, muscle aches and malaise. No serious vaccine-related adverse events were reported. For each primary Covid vaccine, heterologous boosts elicited similar or higher antibody responses as compared to responses to a homologous booster. "These data strongly suggest that homologous and heterologous booster vaccine will increase protective efficacy against symptomatic SARS-CoV-2 infection," the study showed. These interim results cover available immunogenicity data through the initial 29 days following booster vaccination. Investigators will continue to follow participants for one year to assess what impact booster vaccination has on longer-term immune responses. /IANS