Research analyzes the effectiveness of mass vaccination against severe cases of COVID-19

A new study by Fiocruz on the effectiveness of vaccination in Brazil shows that the population has benefitted from the vaccination plan against the Sars-CoV-2 in the prevention of severe cases and deaths by Covid-19, although effectiveness varies between age groups. The analysis, which also included subjects vaccinated with the first dose of the Pfizer shot, was carried out between January 17 and July 19 this year, a total of six months, in which the Gamma variant was predominant. The study evaluated more than 66 million records and is available in the MedRxiv online repository as preprint (without peer review).

The results show a reduction of effectiveness as the age of subjects increases, thus reinforcing the results of previous studies that analyzed the effectiveness of AstraZeneca and Conoravac vaccines (click here to read a study on this subject, by Fiocruz researchers). In the current study, the effectiveness of CoronaVac in the prevention of severe cases among individuals aged 80 years and up was 29.6%, a figure much lower than the estimated rate for those between 60 and 79 years old (60.4%).

Estimated effectiveness in the prevention of deaths was higher among adults between 20 and 39 years old vaccinated with AstraZeneca: 97.9%. This rate is also high (82.7%) among people between 40 and 59 years old vaccinated with CoronaVac.

The evaluation with the Pfizer shot yielded a result of 89.9% protection in the prevention of deaths in the age group from 40 to 59 years old, mostly with the first dose. The study evaluated partially and fully immunized individuals as an ensemble; this is the first Brazilian evaluation study regarding this vaccine.

“Effectiveness also varied in between regions (North, North-East, South-East, South, Center-West), and the effect of reduced effectiveness among older age groups was more pronounced in the South-East and in the South. As expected, effectiveness increases with the second dose, which is why it is important to complete the vaccination”, says researcher Daniel Villela, study coordinator and one of its authors. The research used the SI-PNI and SivepGripe databases and amounted to more than 66 million records in total, including vaccinated subjects and cases, and 65.8 million records of vaccinated subjects, either with one dose or fully vaccinated.

AstraZeneca

When analyzing fully immunized adults, the estimated effectiveness for the AstraZeneca vaccine shows that the prevention of severe cases and deaths ranges from 80% to 90%. According to the age group, this rate may reach 90% efficacy. Effectiveness is slightly lower against severe cases: 79.6% among people between 60 and 79 years old and 66.7% for those above 80.

CoronaVac

The effectiveness rates of the CoronaVac vaccine, also taking into account the full vaccination scheme with two doses, range between 70% and 90%. On the other hand, the estimated rate for severe cases remained 58.4% even among adults between 20 and 39 years old. This rate reached 60.4% for people between 60 and 79 years old and 29.6% for those above 80. Effectiveness in the prevention of deaths was 45%.

Pfizer

The effectiveness of Pfizer ranges from 80% to 90% with partial immunization, i.e. one dose only, for young adults (20 to 39 years old and 40 to 59 years old). Effectiveness with complete vaccination was not evaluated as the number of people fully vaccinated was small in the period under analysis and because this vaccine was introduced to the national immunization plan after CoronaVac.

Methodology

The study evaluates a set of data, with more than 66 million records, consisting of national vaccination bases and cases of Severe Acute Respiratory Syndrome, which includes deaths. Evaluation of effectiveness involves a statistical method that estimates the reduction of risk of an individual developing the severe form of COVID-19 or even dying of it, if they are vaccinated. The method requires rates provided for the number of cases observed in the period as well as the number of people at risk of infection in each of the groups (vaccinated and non), pondered by time.