Delaying second vaccine doses for months and mixing different types of COVID-19 vaccine all seem to be effective, according to findings reported by British researchers.
Delaying the second dose of the Oxford-AstraZeneca COVID-19 vaccine by up to ten months after the first gives as powerful an immune response as a rapidly delivered second dose, the UK experts reported.
A third dose given more than six months after the second also leads to a substantial increase in antibodies and induces a strong boost to immune response against SARS-CoV-2, including variants.
A preprint published yesterday by Oxford University researchers provides some hope for countries experiencing vaccine shortages, they say.
Lead investigator Professor Sir Andrew Pollard, professor of paediatric infection and immunity, said: “This should come as reassuring news to countries with lower supplies of the vaccine, who may be concerned about delays in providing second doses to their populations. There is an excellent response to a second dose, even after a ten-month delay from the first.”
The researchers found that antibody titres increased significantly with a third dose, while T-cell response and the immune response against variants were also boosted.
Lead senior author Associate Professor Teresa Lambe, said: “It is not known if booster jabs will be needed due to waning immunity or to augment immunity against variants of concern. Here we show that a third dose of ChAdOx1 nCoV-19 is well tolerated and significantly boosts the antibody response. This is very encouraging news if we find that a third dose is needed.”
Side effects of the vaccine itself were also found to be well-tolerated, with lower incidents of side effects after second and third doses than after first doses.
The research team said further research is needed to follow up with study participants who received third doses.
Oxford researchers also reported yesterday on trials mixing the Oxford adenoviral vaccine and the Pfizer mRNA vaccine, stating that responses were “robust.”
The study so far analyses doses given four weeks apart and the impact of a 12 week gap is yet to be reported.
Researchers found a double dose of mRNA vaccine gave the strongest antibody response whilst the strongest T cell response came from following an adenoviral vaccine with an mRNA vaccine.
Professor Matthew Snape said: “The results show that when given at a four-week interval both mixed schedules induce an immune response that is above the threshold set by the standard schedule of the Oxford/AstraZeneca vaccine.”
Cunningham AM, Flaxman A, Marchevsky N et al. Tolerability and immunogenicity after a late second dose or a third dose of ChAdOx1 nCoV-19. (AZD1222) Lancet preprint 28 June 2021
Safety and Immunogenicity Report from the Com-COV Study – a Single-Blind Randomised Non-Inferiority Trial Comparing Heterologous And Homologous Prime-Boost Schedules with An Adenoviral Vectored and mRNA COVID-19 Vaccine Lancet preprint 28 June 2021
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