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expert comments about booster doses of the COVID-19 vaccines, including what we know about whether six months is the best gap between second and booster dose

Some general comments from scientists following news that Ministers are considering cutting waiting time for COVID-19 booster vaccines from 6 months to 5.


Prof Jonathan Ball, Professor of Molecular Virology, University of Nottingham, said:

“We are seeing significant levels of virus infection, and eventually some of this virus will find its way to vulnerable people, with the possibility of severe disease and increasing pressure on the NHS.  The impact of vaccines on severe disease can not be overstated – the evidence is clear – people who are fully immunised are far less likely to become ill.  But vaccine immunity wanes over time, and this undoubtedly is contributing to the high levels of virus circulation.  Giving a vaccine boost will help reduce virus circulation and likelihood of disease.

“So the key question is does this boost have to be given after 6 months or can it be just effective after 5 months or maybe even sooner?

“As far as I recall, this was one of the things that the UK’s CoV-Boost trial was going to assess, but I am not aware of the trial data being in the public domain.  Given JCVI advice, one might assume that the 6-month gap gave the best boost, but without seeing the data we can not be sure.  As someone who has spent most of his life studying antibody responses to virus infections, I wouldn’t be surprised if good boosting was seen with a shorter interval.  And the benefits of using a shorter interval – getting the overall levels of immunity raised in the wider population – will undoubtedly slow the virus down and protect more people.  If we have the doses, and the means to administer them, then I think it makes sense to increase the number of people eligible for that boost.  Vaccines really are the best way of keeping on top of this virus.”


Dr Clive Dix, former Chair of UK Vaccine Taskforce, said:

“We know that in the UK the vaccines are protecting against death and severe disease.

“This period last year before vaccination when the number of cases were between 40 to 50 thousands per day we were seeing 1000 to 1500 deaths per day; with the same number of cases now we are recording between 100 and 130 deaths per day, many of which are still unvaccinated.  The vaccines do protect against severe disease and death.  We also know that the antibody levels are declining with time and the question is does that mean the protection is also decreasing.  As the immune response in the elderly is never as strong we need to ensure they are still protected from severe disease and death.  It is therefore essential to vaccinate the elderly with boosters as an insurance policy as the government has indicated, and I believe everyone who is eligible i.e. 6 months on from their second dose should then get the vaccine as soon as possible.  Once we are certain that we have protect the vulnerable in our society we should not be fixated by case numbers.

“I also believe we should increase the campaign to persuade those unvaccinated to be vaccinated.”


Dr Andrew Garrett, Executive VP, Scientific Operations, ICON Clinical Research, said:

“One study that would help inform the discussion around booster doses, is the Oxford Vaccine Group’s Com-Cov trial that compares the AZ and Pfizer vaccines in various combinations for the 1st and 2nd dosing schedule.  This randomised trial compares eight different vaccine combinations – AZ/AZ, AZ/Pfizer, Pfizer/AZ and Pfizer/Pfizer with gaps between the 1st and 2nd doses of 4 weeks and 12 weeks.

“The 4 week data have been reported in June 20211 for immunogenicity and reactogenicity outcomes, but not the 12 week data.  These 12 week data would help inform the debate surrounding the optimal dosing combination and schedule from which a booster program could be judged, and help prioritise those would need boosters earlier.  Although not explicitly answering the booster question, it would provide valuable information on antibody (IgG levels) and T-cell response and indicate whether switching the booster dose, in those that received the same vaccine for their 1st and 2nd doses, might be the optimal approach.  The published 4 week dosing interval data point to a two-dose combination that includes at least one Pfizer dose as producing the highest antibody response, although the 12 week data may show something different.  Seeing all of the data in combination would be very helpful.

“The ONS’s “Coronavirus and the social impacts on Great Britain: 22 October 2021” Bulletin has reported that “around 9 in 10 (91%) adults who have received two doses of a COVID-19 vaccine would be very or fairly likely to have a booster vaccine if offered”.  The more foundational evidence that can be brought into the debate, the more likely an optimal dosing program, with high uptake, can be delivered.”




All our previous output on this subject can be seen at this weblink:



Declared interests

Prof Jonathan Ball: “Receiving funding to develop and trial new generation COVID19 vaccines.”

Dr Andrew Garrett: “I am employed by ICON which is a Contract Research Organization.  ICON provides pharmaceutical services to the pharmaceutical and biotechnology industries. ICON conducts clinical trials on behalf of Sponsors, including COVID-19 vaccine trials. I am a member of the UK Statistical Authority’s (UKSA) Research Accreditation Panel.”

None others received.

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