Booster dose: for whom, when and why they generate so much controversy

In recent months, there has been an increase in the number of infections caused by the highly contagious delta and omicron variants of SARS-CoV-2. In addition, there are indications that the immunity caused by COVID-19 vaccines could wear off over time.

In this scenario, countries that had previously been able to vaccinate most of their adult population now face the challenge of managing the administration of additional doses in the form of third doses and booster doses.

Before getting into the matter, we will remember that this is a complex decision that may not be necessary for most people and that it could help divert much-needed doses to other populations with low levels of vaccination.

The booster dose is not a third dose

First of all, it is important not to confuse third dose and booster dose (or booster, in English).

First, the third dose is the administration of an additional dose to people over 12 years of age with a risk condition due to a compromised immune system. The additional vaccine in this case should preferably be of the same type as the one previously received, that is, one of the mRNA type such as Pfizer or Moderna. According to the Public Health Commission of September 16, 2021, it must be inoculated with a separation of at least 28 days after the second dose.

For its part, the booster dose is an additional injection intended for the general population whose immunity tends to disappear between five and six months after complete vaccination. In these cases, if Moderna is used, half the dose is given and if it is with Pfizer, it is done with the same dose.

According to the Spanish Society of Epidemiology, in its informative note of February 16, 2022, this booster dose is necessary for the entire population over 18 years of age, since it guarantees greater protection against the disease. This is so because the time elapsed since vaccination began has shown that the administration of more than two doses provides long-term protection.

What type of booster dose will you receive?

In relation to the most suitable type of vaccine as a booster dose, there is various evidence in this regard. On the one hand, the UK COMCOV clinical trial showed that the use of a different vaccine from the one used for the initial vaccination schedule could provide greater protection.

On the other hand, in December of last year a clinical trial was carried out in which the immunogenicity of seven different vaccines was compared after the person had received two doses of Astrazeneca or Pfizer. All the vaccines showed acceptable secondary effects, although some schemes caused a greater immunological reaction than others.

Likewise, recent publications of population data from Israel and England have shown that after the administration of the booster dose of Pfizer and Moderna, protection against symptomatic disease increased in people over 50 years of age, regardless of the type of vaccine previously received. .

Therefore, regardless of the vaccination regimen initially administered, the booster dose is a messenger RNA vaccine such as Moderna or Pfizer.

When to give the booster dose?

Based on the available evidence, the booster dose is recommended six months after completion of the vaccination regimen for people over 18 years of age.

For people who have been infected with COVID-19, four weeks after infection was initially recommended. However, following the indications of the Spanish Society of Immunology, it is currently recommended to inoculate it five months later.

The reason is that infection after having the complete vaccination schedule causes a more powerful protective response against the virus.

Will more booster doses be needed?

It depends. Before the advent of omicron, many immunologists thought that a booster dose (or even no booster dose) was sufficient, since it seemed that the majority of the population with the complete vaccine regimen would maintain long-term protection against severe disease and death.

However, with the arrival of this new variant, we have seen that the protection of the vaccine was reduced. On the one hand, given the time that has elapsed since the inoculation of the last dose, the level of antibodies has been decreasing, so a new dose helps to increase these levels.

On the other hand, the presence of 15 mutations in the spike protein of this new variant means that it can escape the immunity of the vaccines developed to date.

So if future variants are highly contagious, as has been seen with omicron, a new dose would provide a wide range of antibodies, some of which will at least be effective.

In fact, according to data from the Centers for Disease Control and Prevention (CDC), booster doses were 90% effective in preventing hospitalization and 82% in preventing emergency room visits during infection with omicron.

Also, a recent study has shown that people with three doses of mRNA vaccine had greater protection against delta and omicron than those with only two doses, although less protection was seen against omicron.

However, on January 11 of this year, the World Health Organization warned that “a vaccination strategy based on repeated booster doses of the original composition of the vaccine is unlikely to be appropriate or sustainable” . The development of new vaccines targeting the new variants will probably be much more effective.

What have they done in other countries?

Most developed countries have already administered the booster dose (figure 1), while many developing countries are waiting for their first dose.

Figure 1. Booster dose of covid-19 vaccine administered per 100 people. /

Our World in Data, CC BY

For example, Israel was a pioneer country in administering the third dose to the immunocompromised population and, later, the booster dose to the rest of the population. In fact, last October it canceled the COVID certificate for those people who had been called to receive said dose and had not done so.

It has also given the fourth dose to immunocompromised people, the elderly and healthcare workers. Thus, preliminary data show that people over 60 years showed twice as much protection against infection and three times more against severe disease, compared to people who had received the third dose at least four months earlier.

For its part, the United Arab Emirates is also among the first countries in the world to offer booster doses.

China, in anticipation of the Winter Olympics, also recommended the booster dose to its population. It has even been a mandatory condition to access certain jobs.

Later, Canada and the United Kingdom authorized the booster dose for the adult population at least six months after completing the first vaccination regimen.

Brazil authorized this dose to all adults over 18 years of age five months after the first vaccination regimen.

And the United States recommends the booster dose five months after the first regimen with Pfizer and Moderna and two months later if the first vaccine was from Janssen.

In the European Union, in October 2021, the European Medicines Agency urged people with a weakened immune system to request a third dose of Pfizer or Moderna vaccines at least 28 days after receiving the second.

It also highlighted that all adults (over 18 years of age) could receive a booster vaccine six months after their last dose, but that it was up to each country to decide eligibility.

Hungary was the first country to administer it and there are notable differences between the various European countries.

Inequity in the administration of vaccines

However, while 72% of the population in high-income countries have received 191.35 doses per 100 people, in low-income countries they have received 18.13 per 100 people (figure 2).

Figure 2. Number of people who have received at least one dose divided by the total population of the country. /

Our World In Data, CC BY

Given the situation, the debate focuses, on the one hand, on whether it will be necessary to continue administering new doses to the general population or to specific groups. On the other, in the ethical dilemma of increasing the number of doses in developed countries, given the limited percentage of the population vaccinated with two doses (or even one) in developing countries .

We have been able to see with ómicron how the fact of not supplying vaccines to countries with much lower vaccination rates has increased the probability that resistant variants will develop. Therefore, we must consider whether the administration of booster doses should be postponed until the vaccines are available to everyone.

This article has been published in The Conversation

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