Popularization of research advances on COVID-19

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A small dose of Moderna is enough to activate an immune-memory response

Infection by SARS-CoV-2 and vaccination are 2 ways of acquiring protective immunity. After infection, an immune-memory response remains active for at least 8 months, deploying antibodies along with CD4 and CD8 T lymphocytes. Pfizer-BioNTech and Moderna, mRNA vaccines have proved very effective in preventing serious cases of COVID-19. In clinical trials of Moderna, 2 doses (100µg and 25µg) were tested, and the stronger of the two was chosen for the final product. Studies on long-term protection have been encouraging in showing only a slight lessening of effectiveness after 6 months. American researchers (at the Center for Infectious Disease and Vaccine Research, La Jolla Institute for Immunology) have nevertheless examined the immune-memory response activated by the smaller dose.

They used the serum of 35 patients who had received two 25µg doses of the Moderna vaccine at different times: 1, 15, 43 and 209 days after the 1st injection. The scientists firstly measured the quantity of antibodies against the spike (the surface protein of the virus). It appears that antibodies are detectable in all patients at least 6 months after the 2nd injection, a result close to that in individuals having been infected by the virus.

They then measured levels of CD4 and CD8 memory T cells specifically targeting the spike. These T cells are present more than 6 months after the 1st injection and are similar in quantity and quality to those present in a previously infected individual. Although the CD8 T cell response is comparable for both dosage levels of the vaccine, the antibody and CD4 T cell response is 2 times greater with the higher dose of the vaccine (100µg). It should be noted that this memory response seems to be reduced in older people, no matter what the vaccine dose. Finally, around 50% of people, even if they are not immunised against COVID-19, possess CD4 T cells capable of recognising SARS-CoV-2 that have been produced in reaction to other benign coronaviruses resulting in colds. In these individuals, the CD4 T cell memory response targeting SARS-CoV-2 increases after vaccination.

In conclusion, the results of this study are encouraging as regards useful dosage levels of the Moderna vaccine. Administration of the vaccine using smaller doses allows vaccine economy, thereby enabling the world’s population to be vaccinated more quickly. It may also be necessary to plan for regular booster shots.

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