Janssen Vaccine: What We Know and What We Don't
Why is it only one dose?
The path to achieving group immunity against COVID-19 does not stop finding stones to avoid. After the suspension of the AstraZeneca vaccine, our hopes were placed on the arrival, scheduled for April 14 in our country, of the Janssen vaccine (a Johnson & Johnson subsidiary), which only requires one dose. However, on the 13th, the pharmaceutical company announced delays in the arrival of its formula in Europe after warning six cases of thrombosis in the seven million vaccinated in the United States.
A week later, the European Medicines Agency (EMA) has given the green light to the vaccine, although it recognizes the possible link of "very rare" cases of thrombi. Salvador Iborra, immunologist at the Complutense University of Madrid, analyzes what is peculiar about this fourth vaccine.
How does the Janssen vaccine work?
The four vaccines approved by the European Medicines Agency (EMA) pursue the same objective: that our immune system makes antibodies that recognize the protein S (spike) of SARS-CoV-2, at the same time that they train T lymphocytes to recognize infected cells. The difference is in how each vaccine achieves that goal.
The Johnson & Johnson vaccine (Ad26.COV2.S.) is based on a human adenovirus, a harmless virus that is used as a vehicle and that carries the genetic information of the coronavirus protein S in the form of DNA, and that is incapable of to replicate. Although adenovirus infects our cells without replicating, the information it carries is translated into messenger RNA similar to that carried by Moderna or Pfizer vaccines.
The greater stability of DNA allows this vaccine to be stored in a refrigerator (up to 3 months), while the previous ones require a more expensive deep-freezing system and not always available.
Why is it only one dose?
Typically, two doses of vaccine are necessary for optimal immune response and maximum efficacy. The second dose usually triggers the somewhat weaker response induced by the first dose. However, it all depends on the information derived from clinical trials.
In the case of the Johnson & Johnson vaccine, it has been tested with a single dose, in which 44,325 volunteers were included ( ENSEMBLE ), and another with two doses (ENSEMBLE2) is being tested. The immunogenicity in both cases has been very good, which would allow predicting that the efficacy will not vary excessively after a second dose. This is a great advantage, given the logistical and production problems that sometimes make it difficult to give the second dose.
How effective do we know it is?
Considering mild and moderate cases of infection, Johnson & Johnson's vaccine, like AstraZeneca's, appears to be less effective than those based on RNA. Furthermore, it varies depending on the vaccinated population (52% in South Africa; 67% in the European population ).
Although it will probably take more time to get solid data, all four vaccines appear to be very effective in preventing serious cases. Even against the new British, South African or Brazilian variants.
Despite their limitations, it is far better to receive any of the four EMA-approved vaccines than to expose ourselves to infection with the virus, as they all similarly reduce the risks of hospitalization or death from the disease.
Therefore, taking into account the logistical problems associated with the different vaccines, we should follow the instructions of the health authorities, and not postpone vaccination thinking that it is better to wait to be able to receive the “most effective” vaccines, or with fewer adverse effects. , especially if there is a high risk of contagion.
Why was the first batch that arrives in Spain destined for the group between 70 and 79?
Clinical trials of the Johnson & Johnson vaccine include healthy participants older than 18 years, and a group (cohort) older than 65. Preliminary results on its immunogenicity suggest that the vaccine is equally effective in different age groups. On the other hand, the serious sequelae of Covid-19 increase with the age of the patient.
In Spain, those over 80 have been the priority group with the Pfizer and Moderna doses (91.4% have received at least one dose) and those under 65 were the target group with the AstraZeneca vaccine. For this reason, 22.3% of people aged 60 to 69 have been vaccinated, while only 13.3% of the population aged 70 to 79 have received at least one dose of any of the vaccines . For this reason, this age group has been chosen for the new vaccine.
What risks or side effects have been reported in your clinical trials?
The Johnson & Johnson vaccine could cause slightly more annoying side effects (fatigue, fever, headache, malaise, etc…) than its competitors, especially in young people with a more “robust” immune system.
There is another important and very rare side effect that will be familiar to us by now, having also been identified in vaccination with AstraZeneca. On April 13, the US Food and Drug Administration (FDA) and the Centers for Disease Control and Prevention (CDC) announced on Twitter that there would be a "pause in use" of the Johnson & Johnson vaccine, "as an extreme precaution measure." "6 cases of a rare and severe type of blood clot in the US" had been detected. in the 6.8 million people who have received the vaccine.
This means that the incidence of these disorders is tremendously low, vastly lower than the likelihood of clotting problems from the infection. However, the manufacturer decided to delay vaccination in the EU on a precautionary basis.
It is thought that the lower efficacy and side effects of the Johnson & Johnson vaccine compared to those based on messenger RNA (Pfizer and Moderna) may be related to whether or not we were infected with an adenovirus before being vaccinated, which may depend of our age.
Our immune system could recognize the adenovirus from the vaccine, reducing its immunogenicity and its effectiveness, this being its main disadvantage.
However, it is not usual for us to know whether or not we have suffered a previous infection with an adenovirus. Therefore, at the moment, we do not have an objective criterion to reject this vaccine. We will have to wait until more information is obtained regarding the very rare adverse cases of coagulation, and vaccination is resumed, as was the case when the AstraZeneca vaccine was stopped for the first time in our country.