# “The ability of vaccines to prevent transmission of the virus decreases”

2021/12/02 Agirre Ruiz de Arkaute, Aitziber - Elhuyar Zientzia Iturria: Elhuyar aldizkaria

Maíra Aguiar is a biologist working on mathematical modeling at BCAM, a research center on applied mathematics. The Basque Government asked them to develop a mathematical model to understand the transmission dynamics of COVID 19 at CAPV. It has analysed its results and considered the current situation.
Ed. BCAM

Thanks to our mathematical modelling we can make reliable forecasts, at the most critical time of the pandemic, for hospitals to be prepared. And the Basque Government asked us to study whether the control strategies established in the CAPV were capable of maintaining the pandemic. In this case, vaccines.

In the midst of an emergency, the development of vaccines had to be very rapid, so its impact on CAPV had not been studied. The correct development of the vaccination process has been evaluated.

What conclusions have you drawn in your research?

This is a theoretical mathematical modeling in which we have taken into account two very different types of txarangas: COWEAK-19 A vaccine capable of preventing severity and preventing infection by the virus and transmission to third parties, as well as vaccination that prevents the development of severe disease but does not interrupt the transmission of the virus.

And another variable we've taken into account is whether asymptomatic people transmit less or more viruses than symptomatic people. It is that, according to a hypothesis, asymptomatic individuals may have the capacity to transmit the virus and, by not isolating themselves, transmit more than isolated symptomatic individuals.

The modeling of all variables shows that the hospitalization rate is considerably reduced in all cases, both with one and the other. But the evolution of the number of copies varies entirely depending on the type of currency. That is, if the vaccine has the ability to stop the transmission of the virus, the number of infected people decreases. However, if the vaccine is unable to stop transmission, the number of infected instead of diminishing increases when asymptomatic people transmit more than symptomatic ones. And it is that being vaccinated can give a sense of tranquility and mitigate the measures, and if vaccinees continue to transmit the virus, it can lead to a worrying situation.

And what is the real ability of vaccines to prevent transmission?

The best would be a vaccine that prevents 100% transmission, but it's not real. In addition, recent studies show that vaccine effectiveness is decreasing considerably. Its ability to avoid hospitalizations is very good and is maintained (even with new variants), but not to prevent transmission.

For example, the vaccine most widely implemented in CAPV, with 90% initial protection against transmission, has been significantly reduced. Therefore, we are detecting positive cases in fully integrated people: they do not need hospitalization, but they are infected and transmit the virus.

What causes loss of capacity to prevent transmission?

We still do not know whether it is due to new variations of the virus, since vaccines do not have the capacity to prevent the transmission of these variants, or whether in time they are losing immunity. But as all variants have reduced protection against infection, immunity may have been lost over time.

After the completion of the clinical trials of the vaccines and the permits for their use, it was not yet clear what protection was given against the infection. Was it not essential to know him to develop effective pre-pandemic strategies?

Yes, of course, very important. But the tests were conducted in the middle of a health emergency and there was a lot of haste. Its effectiveness was detected to prevent the development of the serious disease (more than 70% and WHO requires more than 50%) and permits were granted. It was necessary to protect the health system itself. It was later discovered that they also prevented transmission. The evaluation of vaccines is constant.

And what do you know about the other variable, the transmission capacity of asymptomatic people?

There are still no solid studies to clarify it. There are two hypotheses, both reasonable. There are studies that suggest the transmission of the virus and that, as they continue to live their normal lives, are asymptomatic the main ones responsible for continuing the transmission of COVID 19. But the WHO is not clear. We have therefore taken into account the two possibilities in carrying out the research. It is actually important to clarify this in order to establish effective strategies.

How do you understand the current situation taking into account the results of your research?

Given the current numbers of contagion and hospitalization, we find two hypotheses: that asymptomatic people are more contagious than those really symptomatic (due to their lower viral load but with greater mobility), or that vaccines do not provide sufficient protection to the transmission of new variants of the virus. In fact, in the new viruses we expect a normal evolution that facilitates the transmission of new variants. That would be normal.

In view of the results of your research, what should be done?

Whatever the reason, it is essential not to relax with the measures. Vaccination coverage is above 80% and a high vaccination rate can influence people's behavior: it can be relaxing, both in the use of the kiss and in the maintenance of distance.

We must remember that the effectiveness of vaccines is decreasing in terms of the ability to interrupt transmission, and our research has shown that, in this case, asymptomatic ones can increase the number of positives. It is therefore important to maintain the protective measures. Even more considering that we are in the middle of an age of respiratory infections. We're already seeing double infection. [COWEAK-19 and influenza or other respiratory viruses]. This can make the health of people at risk worse.

On the other hand, since vaccines are losing the ability to stop transmission, it is important to give a booster dose to recover immunity.

Finally, the possibility of introducing children under the age of 12 has recently been put on the table. What do you think?

Vaccination is not an individual action but a social one. Vaccinating children will protect adults. In this case, the vaccine is safe for children and, although its ability to protect against transmission is decreasing, it will always reduce transmission somewhat.

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