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Home Health & Fitness Why cases of Covid-19 in vaccinated people do not mean that immunizing...

Why cases of Covid-19 in vaccinated people do not mean that immunizing is ineffective

By Priscila Pacheco

February 22, 2021, 6:30 pm

Reports of people who fell ill or died of Covid-19 after being vaccinated multiply on social media within a distorted narrative that immunizers are not able to protect someone from the disease. Since January, when vaccines were approved for emergency use in Brazil, posts that raise suspicions about the effectiveness of immunization have accumulated at least 600,000 views and 22,500 shares on Facebook, in addition to circulating on WhatsApp.

But, after all, who has already been vaccinated can contract Covid-19 or infect other people? Are the immunizers in use in the country safe? We explain below.

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Is it possible to get Covid-19 after being vaccinated?

Yes. Vaccines have an overall rate of effectiveness that indicates the ability of the immunizer to reduce the chances of a vaccinee contracting the disease. The rate of CoronaVac, vaccine produced by the Butantan Institute, is 50.38% – that is, those who receive both doses may be 50.38% less susceptible to contamination by the new coronavirus. The Oxford / AstraZeneca vaccine, produced in partnership with Fiocruz (Fundação Oswaldo Cruz), reaches an overall efficiency of 82.4% after the second application.

In both cases, two doses are required for immunization to be effective, and protection does not occur immediately after application. In CoronaVac, the immunity predicted by the overall efficacy rate is usually achieved two weeks after the second dose, which must be applied at an interval of 14 to 28 days after the first, according to a note from the Butantan Institute. In addition, the number of antibodies may continue to increase until one month after the second immunization, depending on the organism.

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Those who take the first dose of Oxford / AstraZeneca may be 76% less susceptible to contracting Covid-19 between the 22nd and the 90th after application, according to a preprint study (not reviewed by other scientists) published in the journal The Lancet in early February. After that period, the second dose should be applied to complete the 82.4% overall effectiveness.

Despite the difference between the overall effectiveness rates of CoronaVac and the Oxford vaccine, it is important that there is no comparison between them. As explained by Laura de Freitas, a microbiologist and researcher at USP (University of São Paulo), rates vary according to the type of vaccine and the research method performed. Aos Fatos explained more about the subject in January.

In addition to the overall efficacy rate and the time it takes to produce antibodies, for immunity to be effective, vaccination coverage must also be taken into account. This is because only when a large part of the population is immunized will it be possible to control the spread of the virus and reduce serious cases. Until the beginning of this week, only 2.79% of the population had been vaccinated in Brazil, according to Fiocruz.

Is it possible to infect someone even though they are vaccinated?

It remains to be seen whether vaccines are capable of blocking the transmission of the virus or not. In the middle of this month, CoronaVac began to be applied massively in the city of Serrana, in the interior of São Paulo, to assess its potential to reduce the rate of transmission of the virus. The results are expected to be released in the second half of May.

The Oxford / AstraZeneca vaccine preprint published in early February states, based on the viral load assessed in volunteers, that the chance of virus transmission was reduced by 67% after the first dose of the immunizer. However, clinical studies are ongoing.

At the moment, while the vast majority of the population has not yet been vaccinated, it is necessary to continue with preventive measures, for example, wearing a mask and avoiding crowds.

Are vaccines safe?

Yes. None of the deaths reported on social media proved to be related to vaccines in use in Brazil, CoronaVac and Oxford / AstraZeneca. In addition, the results of research conducted with the two immunizers did not show a serious side effect.

CoronaVac did not show serious adverse effects in the first two phases of the clinical trial. The study published in the scientific journal The Lancet in November it showed that, in phase 1, the rate was between 13% and 38%, with a predominance of mild symptoms, such as pain at the injection site. As severe and possibly related to the vaccine, only one case of urticaria was reported in which the participant was medicated, recovered and had no other reactions when taking the second dose.

The results presented in the The Lancet on December 8, the Oxford / Astrazeneca vaccine also did not indicate a serious adverse effect or death. During the tests, 175 serious events were observed, but only three of them (hemolytic anemia and myelitis) could be related to vaccination.

This information is also available in the leaflets authorized in January by Anvisa from both CoronaVac and Oxford / AstraZeneca.

Publications on deaths after vaccination against Covid-19 related to other immunizers against Covid-19 have been recurrent in different countries, but there has also been no evidence of any relationship.

Illustration: Luiz Fernando Menezes


1. The Facts (Sources 1 and 2)
2. Butantan Institute
3. The Lancet (Sources 1, 2 and 3)
4. Nature
5. Anvisa (Sources 1 and 2)
6. Fiocruz
7. G1
8. BBC Brazil
9. Deutsche Welle


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