Third dose of the Covid vaccine

The first variant detected in the United Kingdom is now known as Alpha and the last and already the most frequent in our country from India is called Delta. In the first weeks of September 2021, the Delta variant represented 94.9% of the circulating virus in Spain.

The Delta variant of COVID stands out for being highly contagious, practically double with respect to the others and for presenting resistance to the vaccine. This means that in many countries a third dose of the vaccine has been considered.

To date published studies show that current vaccines induce antibody responses that maintain their neutralizing capacity against the new variants.

What is the goal of a third dose?

The goal of the third dose is to reinforce protection of citizens whose weakened immune systems have not responded sufficiently to the first two doses. stimulates the immune system to deal with the protein S of the virus.

The American drug agency (FDA) has endorsed the third dose of vaccine against covid-19 for a part of the population:

• Over 65 years old.
• People with weakened immune systems that implies a greater risk of becoming seriously ill in case of contracting the virus, eg patients with, ...
• Health workers and others people with greater exposure to the new coronavirus like the teachers.

How long does immunity last?

Pharmaceutical companies defend and are interested in applying a third dose of the vaccine. They argue that the nAntibody levels in the blood fall after six to eight months of the second dose.

However, we must know that there are different types of immunity:

• Humoral immunity, represented by the antibodies in the blood that disappears with the months.
• Cellular immunity, represented by cells of the lymphocyte type with long-term memory.

Are they are activated when the virus arrives blocking its proliferation and they end up expelling it, limiting the contagion to a mild disease.

Measuring cellular immunity is complex and hence the difficulty for say yes or no to a third dose. Although it is true that after more than nine months many people have a low or non-existent level of antibodies, it may be that they retain an effective cellular response that lasts for years. Specific T cells stay active much longer than antibodies and this is where the immune memory resides.

In recent months a team of researchers from the Hospital Universitario de Canarias, led by immunologist Yvelise Barrios, has created a new cellular immunity test for covid-19. The test is similar to the Prick, a test used to study allergies on the forearm, or to the tuberculin test for the diagnosis of tuberculosis. Protein S of the virus is inoculated under the skin and if there is cellular immunity, redness and inflammation appear at the puncture site in the next three days. The test is safe because it does not inoculate viruses, it can be used both in vaccinated people and in those who have passed the infection and so far it has not given false positives.

A positive result would incline us to discard a third dose when we appreciate the body's defensive reaction.

What data does Israel provide on the third dose?

Israel has been the first country to administer the third dose vaccine and has already been vaccinated with booster doses to three million people.

So far, vaccinated people have not registered serious side effects after this injection and they have been described milder reactions than with the second puncture against the coronavirus highlighting pain at the injection site, headache and tiredness or fatigue.

Their results reflect that the third dose has reduced the rates of contagion and serious infections in one month among those over 60 years of age, lowering the risk of contagion by 50 to 5%, although long-term studies are lacking.

What if the immune response is not achieved?

In people who do not achieve the desired immune response, you are testing an investigational therapy: monoclonal antibodies. Monoclonal antibodies are man-made proteins that act like human antibodies in the immune system that are being shown to reduce the risk of developing symptomatic covid-19.

For an end to the pandemic

The main risk of contagion from COVID remains lunvaccinated population, even in areas with high vaccination rates.

Vaccination against covid-19 in the most developed countries is well advanced, but for example in India and Iran only 11% of their inhabitants have been fully immunized, in Bangladesh 5%, in Senegal 3% and in Algeria 6%. In Kenya or Ghana they barely exceed 1% with the full pattern.

The virus can generate more resistant or lethal variants in the unvaccinated population. This is why it is important prioritize vaccination globally versus the administration of a third dose in a healthy population that has already received the vaccine.

"Although the third dose may provide some advantage, the benefits of immunizing the unvaccinated are much greater", highlights a work published by The Lancet, since "it can accelerate the end of the pandemic, and will prevent more variants of the coronavirus from evolving ”.

• The aim of the third dose is to reinforce the protection of citizens whose weakened immune systems have not responded sufficiently to the first two doses.
• Although it is true that after more than nine months many people have a low or non-existent level of antibodies, it may be that they retain an effective cellular response that lasts for years
• The virus can generate more resistant or lethal variants in the unvaccinated population. For this reason, it is important to prioritize vaccination worldwide over the administration of a third dose in a healthy population that has already received the vaccine.