The possible second dose of Janssen's vaccine has been in the air since they started vaccinating. Just look at the fact that the pharmaceutical company started a clinical trial using two doses of the vaccine as early as November 2020.
This should come as no surprise because we have already seen that with all other vaccines, two doses are necessary to obtain better protection. Other adenovirus-based vaccines, such as Astrazeneca and Russia's Sputnik V, use two.
As expected from the results obtained during clinical trials, the data indicate that the effectiveness of Janssen's vaccine in preventing hospitalisations and deaths from covid-19 appears to be lower than that observed with the other two-dose vaccines.
Preliminary results indicate that neutralising antibody levels increase several-fold and protection increases to levels similar to those observed with the other vaccines.
Although these are preliminary results and it is best to wait for the full study data, it seems clear that, although one dose of Janssen's vaccine protects against covid-19, two doses protect better. Therefore, the debate now is whether this improved protection justifies the administration of a second dose or not.
One thing we cannot forget is that vaccines also limit infections and transmission of the virus. Therefore, although the protection obtained with one dose may be less than that obtained with other vaccines, the fact that a large part of the population is vaccinated means that this protection may be sufficient.
At the present time, it seems logical to consider the need to administer a second dose of the Janssen vaccine in order to obtain the maximum possible protection, at least in the most vulnerable people and those in risk groups.