Why Some Countries Are Considering Delaying Second COVID Vaccine Doses

A person receives Pfizer's COVID-19 vaccine at the Amedeo di Savoia Hospital in Turin, Italy on December 27, 2020. MikeDotta/Shutterstock.com

At long last, the COVID vaccines have arrived with a whole host of countries starting to roll out doses to those most at risk. While getting to this stage was no small feat, scientists and health authorities are now faced with yet another gigantic step: getting the vaccine to the public. 

Most countries will not be instantaneously in possession of enough doses to give the jab to their entire population, nor vaccinate those people most at risk, let alone have the means to store the vaccines and the trained personnel to administer them. Faced with the prospect of short supplies and logistical problems, many countries are faced with tough choices about how to distribute the two separate doses needed for most COVID-19 vaccines.

Some health authorities have toyed with the idea of dishing out the first dose quickly to as many people as possible and delaying the second dose. The idea is that this would be a speed-over-efficacy trade-off: a single dose of the vaccine will not be as effective for an individual, but at least more people will have some degree of protection, thereby slowing down the overall spread of infection in the population. However, many are not sold on the idea and consider this move to be a gamble. 

The UK government’s advisory committee has argued it should take this path, noting that “given the high level of protection afforded by the first dose, models suggest that initially vaccinating a greater number of people with a single dose will prevent more deaths and hospitalizations than vaccinating a smaller number of people with two doses.” 

Not everyone in the UK agrees with this, though. The British Medical Association has said the decision to delay second doses is “unreasonable and totally unfair," adding it could cause “huge logistical problems.”

There has also been talk of a similar plan to delay the second dose in the US, although the Food and Drug Administration (FDA) has outright said it's against the idea, arguing there is a concerning lack of evidence about how delaying second doses might affect the efficacy of the vaccine. 

Although all of the approved vaccines are most effective after two doses, they do appear to confer some immunity after the first dose. A study in the New England Journal of Medicine found that the efficacy of the Pfizer-BioNTech vaccine between the first and second doses was 52 percent. However, the UK's Joint Committee on Vaccination and Immunisation (JCVI) says the first dose of the Pfizer-BioNTech vaccine is calculated at around 90 percent effective after the first dose allowing for the immune response to develop for a couple of weeks. The JCVI adds: “Similar findings were seen with the Moderna mRNA vaccine out to 108 days after the first dose.” As for the Oxford-AstraZeneca vaccine, the efficacy after the first dose to the time of the second dose was around 70 percent.

A trio of new studies published this week in the journal Annals of Internal Medicine (here, here, and here) have also suggested that the single dose is not ideal, but it’s better than nothing. They argue that a single-dose vaccine with 55 percent effectiveness may confer greater benefit, in terms of a population-wide scale, than a 95 percent-effective vaccine requiring two doses.

None of this is ideal, obviously, but the clock is ticking. Given the high stakes, many health authorities across the world are likely to be faced with these kinds of decisions. Most scientists are simply hoping the decisions and strategies being made are guided by and backed up by data and evidence.

“Often, decisions to safeguard public health have to be taken in the face of uncertainty. Importantly, however, when robust evidence can be got to evaluate those decisions, we should ensure that robust, randomized evaluation takes place from the outset,” Professor Sheila Bird, formerly Programme Leader at the MRC Biostatistics Unit in Cambridge, commented this week. 

Updated 01/06/2021: The WHO's Strategic Advisory Group of Experts on Immunization (SAGE) says it recommends two doses of Pfizer COVID-19 vaccine within 21 to 28 days. However, they added that the second dose could be delayed under “exceptional circumstances.”

For more information about COVID-19, check out the IFLScience COVID-19 hub where you can follow the current state of the pandemic, the progress of vaccine development, and further insights into the disease.


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