Vaccination does not reduce the risk of transmission of the Delta variant of SARS-COV-2, the predominant strain of the virus responsible for covid-19. A vaccinated person has, at the peak of the infection, a viral load similar to that of a person who has not received the vaccine. However, this decreases more quickly making the person contagious for less time.
The conclusion is from a new study released late last week by the scientific journal The Lancet. Over a year, researchers tracked 621 people in the UK.
The research concludes that, while the probability of those infected transmitting the disease to others is similar between vaccinated and unvaccinated (25% and 23%, respectively), vaccination has a significant impact on the probability of contracting the virus. In the domestic context, only 25% of vaccinated contacts were infected compared to 38% of unvaccinated ones.
“Before this work there were others who suggested this, but it is really a surprise compared to what we thought before. We were expecting that, even if the person was infected, the viral load would be lower in those vaccinated”, says Paulo Paixão.
In other words, if someone in your family or closest circle is not vaccinated, your vaccination will protect you, but it does not guarantee the protection of others. Even if you are vaccinated, the risk of transmitting the disease is as high as if you had not been previously vaccinated.
“This may seem like a detail, but really from a public health point of view it can be important. It's another blow in the history of group immunity”, says the president of the Portuguese Society of Virology.
The study itself demonstrates that "vaccination alone is not enough to prevent transmission." “That's why we keep the rules”, says Paulo Paixão.
For the specialist, this data does not mean, however, that it is not worth taking the vaccine. “The impact of vaccination on the severity of cases, and mortality, is massive,” he said.
“Cases will increase now with winter. Mortality is not going to be anything like what happened last year, but a series of cases will appear. What we think is that mortality will rise mainly at the expense of the unvaccinated. The virus will continue to circulate and effectively the unvaccinated ones will have a much more significant risk than what was initially thought”, he summarizes.
Also a professor at the Faculty of Medical Sciences at Universidade Nova de Lisboa, Paixão explains that the impact of the disease will not be so great in Portugal because "the groups at higher risk had massive adherence to vaccination", but "are still at risk until the end of winter - we have to be careful”.
The Lancet study does not look at infections related to the type of vaccine given to individuals. But for Paulo Paixão, “it is possible” that the type of vaccine affects these numbers, but “there is still no clear evidence about this”.
This idea is in line with what the professor of public health, Tiago Correia, had already told the press. In the United Kingdom, the predominant vaccine was AstraZeneca, while in Portugal it was Pfizer and Moderna, which may partly explain the differences between countries.
Regarding the“ideal way to test”, to detect whether the person can still transmit the virus - the current belief of the scientific community is that the antigen test is more effective in detecting transmissible cases because, being less sensitive, it detects the highest viral loads. Thus, a person with a positive PCR test but an antigen negative one is no longer able to transmit the virus.
However, an article released this week also in "The Lancet" demonstrates that "this is not a perfect tactic", explains Paulo Paixão, as he detected cases with negative antigen that were still transmitting.
The “most indicated” test to determine the potential for contagion is cell culture, which “studies the viability of the virus” - that is, “if it grows in culture, it is still infectious.” However, “it is very complicated to do and only laboratories can do. It is not practical to be used."
Thus, “we do not have any test that assures us in a very reliable and simple way whether the person is transmitting the disease or not”, and which could easily be applied before a patient is discharged."
Original article available in Portuguese at http://postal.pt/
