A study conducted by Brazilian researchers shows how age influences the effectiveness of the two main immunizers used in the country against COVID-19. Submitted as a preprint in MedRxiv, the study Influence of age on effectiveness and duration of protection in the Oxford/AstraZeneca and CoronaVac vaccines involved more than 75 million immunized people, becoming the largest study conducted with the two immunizers and may serve as a basis for guiding public health decisions, including the need for additional doses or booster doses.
Coordinated by Manoel Barral-Netto, a researcher at Fiocruz Bahia, the study evaluated the immunizers effectiveness in 75,919,840 people vaccinated in Brazil between January 18 and July 24 this year. The results show that both vaccines are effective in protecting against infection, hospitalization and death, considering the complete vaccination scheme (two doses): AstraZeneca/Fiocruz, with 90% protection, and CoronaVac with 75%. The research also showed that both vaccines offer protection against moderate and severe cases against the variants of concern circulating in Brazil during the analysis period.
However, when separating the vaccinated groups by age, the data shows that there is a reduction in protection as the age increases and that the two vaccines offer different protection degrees with the full vaccination schedule. From 80 to 89 years old, the AstraZeneca/Fiocruz vaccine had a protection rate against death of 89.9%, while CoronaVac had 67.2%. Over the age of 90, these rates were 65.4% in those vaccinated with AstraZeneca/Fiocruz and 33.6% with CoronaVac.
"We were already suspicious of the influence of age on the decline in effectiveness because the same thing happened with other vaccines. What we have done is clearly delineate that decline point. This is also the first comparison made between vaccines that use different platforms", said Barral-Netto. "The intention is to provide data on which to base decisions for managers.
Importance of research in Brazil
AstraZeneca/Fiocruz has already been approved in 181 countries and CoronaVac in 39. However, few nations can offer such a broad database for a study of this size. Although the 211 million Brazilians are divided into different regions, the health information collection system is the same, providing a wide source of data — which allowed the analysis by age groups. For this, data from the National Immunization Program (PNI in the Portuguese acronym), from e-SUS-Notifica, and from the Influenza Epidemiological Surveillance Information System (Sivep-Gripe) were used.
Overall effectiveness after complete vaccination scheme
Individuals who received AstraZeneca/Fiocruz vaccine both doses had 72.9% protection from infection, 88% protection from hospitalization, 89.1% protection from ICU admission, and 90.2% protection from death.
People with the full CoronaVac vaccine regimen had a 52.7% infection; 72.8% hospitalization, 73.8% going to the ICU, and 73.7% death lower risk.
Effectiveness according to age group
When age is considered, vaccines offer different protection levels, with evidence of an increase, albeit at different levels, in the vaccine hospitalization incidence rate according to age. The AstraZeneca/Fiocruz complete vaccine scheme induced an effectiveness rate of about 90% in different outcomes up to age 89. In the group over 90 years, a reduction in protection levels was observed, with effectiveness against death of 65.4%.
In the CoronaVac vaccine case, after the age of 60, a downward trend in the overall effectiveness of 75% is observed, evidenced in every decade of life analyzed, with this decrease being more sensitive in the group over 80 years old and reaching an even greater impact in the population over 95 years old, where the effectiveness against death drops to 33.6%.
The data highlights the "critical impact of age on the effectiveness of two vaccines that employ different technologies", says the text.
Previous studies
The research also shows that the protection afforded by CoronaVac against symptomatic COVID-19 is compatible with previous efficacy studies conducted in Brazil, but lower than work done in Turkey. In Chile, the effectiveness levels for infection and hospitalization were higher than in Brazil, which could be partially explained by the higher proportion of younger individuals immunized with CoronaVac in Chile (51.2% of immunized individuals under 60 in Chile and 38.5% in Brazil). The study also recalls that the collapse in the Brazilian health system, the vaccination speed, and the difference between the variants circulating in the two countries may have influenced these differences.
In relation to AstraZeneca/Fiocruz, the study shows 72.9% effectiveness against infection — above the 66.7% registered in a combined analysis of clinical trials conducted in the UK, South Africa, and Brazil. Effectiveness against hospitalization, meanwhile, is consistent with the 80% and 88% observed in studies in Scotland and England, respectively. "Furthermore, our findings support the high level of protection offered by Vaxzevria [as the vaccine is also called] despite the high circulation of the Gamma variant in Brazil during the period", the text reads.
Third dose
According to the study, the reduced effectiveness may be related to the difference in technological platforms used by the vaccines and their impact on immunogenicity, as well as to a natural process of lower immune response in older individuals, called immunosenescence. For the researchers involved, in a context where there is limited vaccines availability, being able to identify more precisely the age limits at which immune protection is compromised becomes valuable evidence for implementing public health measures.
"Considering the current scenario in Brazil, our findings demonstrate the eventual need for a vaccine booster dose in individuals above 80 years of age who received CoronaVac and in those above 90 years of age immunized with AstraZeneca/Fiocruz", says the study.
The research results were presented to the Ministry of Health and the group of vaccine experts from the World Health Organization (WHO). Researchers from the Gonçalo Moniz Institute (Fiocruz Bahia); the Center for Integration of Data and Knowledge for Health (Cidacs/Fiocruz Bahia); the Federal University of Bahia (UFBA); Fiocruz Brasília; the University of Brasília (UnB); the Federal University of Ouro Preto (UFOP); the University of São Paulo (USP); the State University of Rio de Janeiro (UFRJ); and the London School of Hygiene & Tropical Medicine participated in the study.