What is the scientific rationale for the postponement of the second Covid vaccine dose?

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The UK health authorities have agreed, triggering international concern, to delay the administration of the second dose of the Covid 19 vaccine and also allow the combination of doses from different manufacturers. What is the scientific basis for this ruling? Why did the UK conclude that the interval between the first and second doses of the vaccine should be extended? The initial plan was to deliver the first vaccine to priority classes, followed three weeks later by a second dose.

However, a rapid rise in the number of 19 cases of Covid, combined with the advent of a more readily transmitted strain and concern about the availability of stocks of vaccines, led the Joint Committee on Vaccination and Immunization (JCVI) to explore other options.

It was determined that if the administration of the second dose was postponed until 12 weeks later, initial vaccination could enter more weapons more rapidly – by which time it is anticipated that much more vaccine doses would be available. A statement from the JCVI said, “It is very likely that this will have a greater public health impact in the short term and reduce the number of preventable deaths from Covid-19,” Isn’t this the same strategy suggested by Tony Blair? The former prime minister proposed offering as many individuals as possible a single dose of Covid vaccine, rather than maintaining stocks to allow for a second injection, which is very different from what is happening now. The JCVI emphasized that everybody would obtain a second vaccine with a cumulative delay of 12 weeks between the first and second doses, which would only be longer than originally expected. This second “booster” dose is likely to increase and optimize the immune response period, resulting in longer-term safety. Why are some researchers worried about this? Mainly since, in clinical trials, the results of extending the two doses have not been checked. Neither has a first dose of Pfizer/BioNTech vaccine administered followed by a second dose of Oxford/AstraZeneca vaccine or vice versa, which is recommended by the JCVI only in ‘exceptional cases,’ such as when no second dose of the original vaccine is available. The biggest concern is that the immunity of individuals would drop off before getting their second dose, exposing them to the risk of Covid-19, but the risk will still be smaller than if they had not received the vaccine and would be increased after receiving their second injection. Pfizer noticed that only two doses were administered 21 days apart in the study – much less than 12 weeks apart. A British Society of Immunologists consensus, however, notes that delaying booster vaccination by eight or nine weeks is unlikely to make any difference in the long run. What about the combination of different manufacturers’ doses? Again, this has not been evaluated in clinical trials, while British and Russian scientists are investigating whether the combination of Oxford/AstraZeneca and Sputnik V vaccine vaccines could provide better safety than two doses of the same vaccine. In principle, an efficient immune response can still be elicited by mixing doses from different manufacturers, because almost all of the Covid 19 vaccines produced are based on the same viral “spike” protein, even though it is presented in slightly different ways to our bodies. May other countries follow the UK? Potentially. The World Health Organization said that there was no empirical evidence to support a delay of more than six weeks in the administration of the second dose of the Pfizer/BioNTech vaccine, but it is understood that Canada and Germany are considering it, and Denmark said it would accommodate a six-week gap so that a first vaccination could be offered to more individuals. However, U.S. health authorities have ruled the possibility out.

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