The Centre for Disease Control in the USA has stated that for both mRNA vaccines (Pfizer/BioNTech & Moderna) “The second dose should be administered as close to the recommended interval as possible” – i.e. The MHRA has approved and international expert bodies have advised using the existing study dosing schedule. mRNA vaccines have never been used in late stage human trials before 2020 and the only data we have are from the Pfizer/BioNTech and the Moderna (another mRNA vaccine) studies, which use different vaccines at different concentrations.įurthermore, given that the Pfizer/BioNTech vaccine is scheduled to be delivered to 3-10 million vulnerable adults or health and social care workers exposed to high levels of risk, the JCVI/CMOs appear to be advocating what could prove to be a major change with attendant clinical risk (eg less efficacy than that generated by Pfizer/BioNTech study). The time interval for a mRNA booster may be very critical for getting the best sustained immune response. Human studies are urgently needed, ideally before pursuing the delayed 2 nd dose strategy. This raises concerns that extending a 2 nd injection out to beyond 28 days could compromise vaccine efficacy. However, the specific anti-viral immune response was strongly boosted with a 2 nd injection of vaccine. Animal models using LNP-mRNA vaccines against the virus show that ~28 days after a 1 st injection, correlates of virus neutralising antibody production fall off markedly (suggesting limited survival and stimulation by the vaccine mRNA and its’ encoded Spike immunogen). How well the LNP-mRNA survives after a 1 st injection and induces durable human immune responses is unknown. Modifications have been introduced, to improve the delivery and survival of the mRNA vaccines, including as in the Pfizer/BioNTech vaccine, enveloping the mRNA in lipid nanoparticles (LNP-mRNA). Non-replicating mRNA (basic structure used in the two covid-19 vaccines- Pfizer/BioNTech and Moderna) after injection into the body becomes quickly degraded by extra and intracellular enzyme systems (RNAases). A crucial point is that the Pfizer vaccine uses mRNA. There are also no data on how long a first single dose of the mRNA generated immunogen (the virus Spike protein) induces a clinically effective immune response, comprising T cells, B cells, and their memory cells. While assumptions can be useful for generating a hypothesis, alone they are not a sufficient reason to alter a known effective dosing regimen. The JCVI advice and the CMO’s decision to delay the second dose to between 4-12 weeks is not based on data from the trial, but on an assumption of what would have happened if the second dose hadn’t been given at 21 days. Sub-group analyses confirmed similar vaccine efficacy among subgroups including patient age. The overall results showed an efficacy of 95% reduction in covid-19 cases at least seven days after the second dose, which was delivered to a scheduled 21-day interval between the 1 st and 2 nd dose. It was well powered, reporting on 37,706 individuals at 152 sites in six countries who received vaccine or placebo and had median follow-up of 2 months. The study was a well-designed, prospective randomised controlled trial with blinded endpoint assessment. The Pfizer/BioNTech vaccine is the first human mRNA vaccine evaluated in a phase 3 clinical trial. This is disputed by vaccine manufacturers. The proposal has been supported by the UK Chief Medical Officers (CMOs) who indicated there are vaccine shortages across the UK. Due to the accelerating pandemic and a desire to maximise the numbers in the population to receive a first dose vaccine, the Joint Committee on Vaccines and Immunization (JCVI) has proposed changing the dose schedules by considerably extending time to the second booster dose. Two covid-19 vaccines, (from Pfizer/BioNTech and AstraZeneca/Oxford) recently approved for emergency use in the UK have a defined time scheduled two doses for use. Equally critical is that vaccines’ proven to be effective in a particular dosing schedule are not altered without solid scientific support or evidence. The importance of successful vaccination strategies in controlling the covid-19 pandemic cannot be overstated and should be vigorously endorsed.
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