Effect of vaccination against Covid-19 one year after its introduction in Brazil

The present study estimated the population effect of vaccination against Covid-19 in Brazil, more than a year of implementation of the NOP, analyzing the association of vaccination coverage with the temporal trend of morbidity and mortality indicators. Two population groups (people aged 18 to 59 and aged 60 years or older), were analyzed by comparing temporal trends in the pre- and post-vaccination periods.

In summary, our findings indicate that vaccination against Covid-19 had an influence on the reduction of hospitalizations and deaths from the disease in Brazil. The hospitalization rate and mortality per 1 million inhabitants had a strong inverse association with vaccination coverage. This association was statistically significant in both groups analyzed. In contrast, vaccination had less influence on severity and case fatality rate.

The most intense epidemic wave of Covid-19 was observed in the age group of 60 years or older and occurred during the predominance of the Gamma variant, eight weeks after the start of vaccination against Covid-19 in the country (EW 11/2021). In that period, vaccination coverage for the first dose was 26.8% and for the second dose it was 6.0%, and there was still no official recommendation for the booster dose.

Therefore, the third epidemic wave, observed in the same group of people, occurred exactly one year after the start of vaccination against Covid-19 (EW 3/2022), when vaccination coverage for people aged 60 years or older was 94, 6.0% for the first dose, 93.7% for the second dose and 62.8% for the booster dose. At that time, the circulation of the Omicron variant predominated in the country.

However, the regression models in this study demonstrated a statistically significant effect in reducing the risk of hospitalizations and deaths from Covid-19 with the increase in vaccine coverage in Brazil. In the case of people aged 60 years or older, for example, with each weekly percentage increase in vaccination coverage, the hospitalization rate was reduced by 0.02% (IRR: 0.982), the case fatality rate at 0.01% (IRR: 0.997) and the mortality rate at 0.02% (IRR: 0.983).

A study that evaluated the initial impact of vaccination on the Covid-19 pandemic in the United States of America (USA) showed that vaccination also significantly slowed the increase of cases and hospitalizations for the disease. It was demonstrated that an additional increase of one person vaccinated per 100 inhabitants (with two doses) reduced by 1.1% the weekly incremental rates of cases and hospitalizations Based on these estimates, vaccination reduced the number of new cases by 4.4 million and by 0.12 million cases hospitalizations in the US initially [22].

Another study showed that in six countries (Israel, United Arab Emirates, Chile, Hungary, Qatar and Serbia), where at least 50% of their population had been vaccinated, peaks of Covid-19 occurred after the start of vaccination and before obtaining herd immunity. However, they concluded that vaccination against Covid-19 contributes to the reduction of cases of the disease, ranging from 1.46 to 50.91%, and their models explained the variability of outcomes from 57.2% to 89.9% [23].

This condition may have been aggravated by the circulation of highly adapted and transmissible variants, such as Delta and Omicron, and by the low coverage of the booster dose whose objective was to boost the immunity of those who had been earlier vaccinated against Covid-19, especially in older people and those with comorbidities.

VOCs or VICs invariably present greater virulence or transmission capacity, and may even escape the immunity already acquired (via vaccine or natural infection), contributing to decrease in the effectiveness of COVID-19 vaccines [24,25,26].

In the UK, a study demonstrated that the effectiveness of COVID-19 vaccines for the Delta variant compared to the Alpha variant was reduced by 37% for the first dose and 6 to 10% for the second dose [27]. Similar results were found in relation to the Omicron variant [25].

Our study showed that vaccination coverage for booster doses was incipient and had slow advancement in Brazil. Although the supply and availability of vaccines has progressively increased, vaccine hesitancy and the feeling of security by the population brought about by the rapid decrease in cases may be related to low adherence to booster doses [11, 28].

With decrease in the effectiveness of vaccines against new variants, the vaccination coverage needed to achieve herd immunity increases [23]. A study conducted in the USA showed, for example, that for a vaccine with an effectiveness of 80%, at least 82% of the population should be immunized to achieve herd immunity to the point of reducing deaths from Covid-19 [9].

The efficacy of the vaccines used in Brazil, before the circulation of the Delta and Omicron variants, ranged from 54 to 95% to prevent infection with SARS-CoV-2 and from 67 to 76% to prevent moderate to critical cases of Covid-19 [15,16,17,18]. To achieve the much-desired herd immunity in Brazil, vaccination coverage must be high and maintained in all population groups.

To the best of our knowledge, this is the first published study that used this method to analyze the population effect of vaccination against Covid-19. In this regard, this study contributes to findings of national and international interest, especially when considering the importance of a country of continental proportions like Brazil.

However, it should be noted that the method applied in this study has already been used by other researchers to analyze the population effect of vaccination on the control and prevention of other vaccine-preventable diseases, such as pneumococcal disease and chickenpox, for example [29, 30].

The interpretation of the results of this study must consider some limitations imposed, mainly, by the use of administrative data that may have underestimated the analyzed indicators, especially vaccination coverage, which depends on timely registration of disease cases and also the vaccinated cases in their respective information systems.

It is worth highlighting that individual data were not analyzed while comparing the rates among vaccinated people in relation to those who were not vaccinated, stratified by the number of doses received. Thus, it is important to note that the statistical association observed in this study does not reflect the individual effect of vaccination against Covid-19.

Finally, it should be considered that other factors, such as non-pharmacological measures – use of masks, social distancing, mass testing, among other interventions – may also have played an important role in controlling the pandemic in Brazil.

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