Release N°50/2021 COVID-19, the efficacy of mRna vaccines continues to be high

ISS, Ottobre 6th 2021

Seven months into the vaccination programme and there has been no decline in the efficacy of the mRna vaccines in reducing the Covid-19 infections in the general population, while there has been a slight drop in protection from infection (symptomatic and asymptomatic) in some specific groups. This was stated in the fourth report of the Working Group "COVID-19 vaccine surveillance" of the Istituto Superiore di Sanità –ISS (Italian National Institute of Health) after analysing the data from the COVID-19 Integrated Surveillance System and from the national vaccine registry. The data pertaining to more than 29 million people who received at least one dose of the mRna vaccine were examined and followed up until 29 August 2021. Efficacy was evaluated by comparing the incidence of infections (symptomatic and asymptomatic), hospitalizations and deaths at different time intervals after the second dose versus the same incidence observed in the 14 days following the first dose, considered as baseline or control period'.

These are the main conclusions
-    In the general population seven months after the second dose there is no significant reduction in efficacy in terms of protection from infection (symptomatic and asymptomatic), which remains at 89%. Also the protection against hospitalization and death remains high (96% and 99%) at six months after the second dose.
-    In immune-compromised people, a reduction in the protective effect against infection is observed starting from 28 days after the second dose. The estimate, in this case, presents high variability due partially to the small number of individuals included in this group but it is also connected to the diverse pathologies present in this category.

-    In people with comorbidities there is a reduction in protection from infection, from a 75% reduction in risk after 28 days from the second dose to 52% after about seven months.
-    The effectiveness against infection in people over 80 and in the guests of long-term care facilities decreased slightly, while remaining above 80%.
-    Comparing the data referring to the period from January to June 2021, the period in which the Alpha variant predominated, versus the data referring to the months of July and August, when the Delta variant prevailed, there was a reduction in efficacy against infection from 84.8% to 67.1%. On the other hand, the effectiveness of the vaccine against hospitalization remained high (91.7% versus 88.7%). The apparent reduction in the effectiveness of the vaccines in preventing infection could be due to the time elapsed since vaccination and / or to a decreased effectiveness against the Delta variant. Changes in behaviour following relaxation of other preventive measures (mask wearing, physical distancing) may also have contributed.
-    The data are consistent with those obtained from the comparison versus unvaccinated individuals, published weekly in the full report of the ISS. It is essential to go on providing these updates and to continue the monitoring also in the coming months.