August 1, 2021

Why a person vaccinated against COVID-19 can spread the virus



  • A vaccinated person is protected against the disease but not against the virus.
  • When we become infected we can be asymptomatic carriers of SARS-Cov2 or sick with covid-19, and in both cases, transmit the virus to people who may be carriers or become ill.
  • Only herd immunity protects us against the disease and also against the virus.

To spread is to transmit the virus, not the disease

Viruses, bacteria and other microorganisms are part of nature, even our bodies. Without going any further, there we have the human microbiome, which is what the set of microorganisms that inhabit our body is called, forming different ecosystems that fulfill vital functions for our proper development and well-being.

In addition, we constantly come into contact with viruses and bacteria that are not common in our body. Sometimes they can colonize or occupy a certain ecosystem temporarily or permanently without causing any damage, while others cause an infectious disease. Whether we are colonized or infected we can infect other people, it does not matter if we have become ill or not (asymptomatic carriers). The disease will depend on many factors, but fundamentally on the ability of our immune system to recognize the foreign agent (virus, bacteria, etc.) and fight against it. This capacity is less when contact occurs for the first time. However, when the immune system already knows the “aggressor”, its ability to fight it is much greater.

This is precisely what vaccines are responsible for: showing us the microorganism or a part of it in a safe way to increase our immune capacity and avoid disease.

Vaccines are safe and prevent disease

When a person is vaccinated with any of the vaccines approved by the European Union To cope with the current pandemic, it protects against the covid-19 disease.

These vaccines, regardless of design (inactivated or attenuated virus vaccines, protein-based vaccines, viral vector vaccines, DNA or RNA vaccines, etc.) share a common goal: to teach our immune system to recognize and block safely SARS-CoV-2 virus to prevent covid-19 disease. This recognition and defense occurs through antibodies and other cells of our immune system.

The first thing to be clear about is that vaccines against covid-19 are safe, as explained by the Spanish Agency for Medicines and Health Products (AEMPS).

Second, the immunization process is not immediate. The immune system takes time and, therefore, a recently vaccinated person could become infected (sick or not) and infect.

Finally, once individual immunity has been achieved, in the case of vaccines marketed today, it appears that sterile immunity is not achieved. That is, the replication of the virus in the mucosa of the respiratory tract is not completely inhibited. Consequently, it is possible to infect other people.

Achieving sterilizing immunity is the goal of Luis Enjuanes, who leads a research group at the National Center for Biotechnology (CNB-CSIC) where a new intranasal vaccine. This vaccine would stop the spread of the virus by preventing it from inhabiting the respiratory ecosystem. It is a new model that takes time. While it arrives, available vaccines are needed to curb the pandemic.

Why herd immunity is so important

And it is very possible that now someone is wondering: Does that mean that vaccines do not work? Why do we get vaccinated then?

They are logical questions. Access to information is a necessity that enhances public trust and an ethical obligation, but sometimes carries a risk of misinterpretation.

It is very important to know that, even if sterilizing immunity is not achieved, vaccines are effective because they reduce the risk of infection and, consequently, the probability of transmitting the virus to other people. That means fewer infections, less disease, fewer sequelae and fewer deaths. This reduction in risk is necessary as an essential measure to fight the pandemic.

When a person is vaccinated, the risk of infection and the probability of transmission are reduced by X percent. The way to further reduce this probability is by increasing the number of people vaccinated. As simple as that.

Group immunity or herd immunity is the protection that is achieved when the population becomes immune. In this situation, the risk of contracting the disease is very low and, therefore, transmission is minimal. Something fundamental if we take into account that, in addition to reducing transmission, some people cannot be vaccinated because they have some clinical contraindication (for example, severe allergy to some component of the vaccine) and this herd immunity protects them.

In Spain we have not yet achieved group immunity. It is estimated that it will be achieved when 70% of the population is immunized. The last Covid-19 vaccination process activity report from the Ministry of Health, dated June 9, 2021, shows us that 31,004,060 vaccines have been administered (Pzifer, Moderna, AstraZeneca and Janssen); that there are 20,305,788 people vaccinated with one dose (42.8% of the population); and that there are already 11,552,393 people with a complete pattern (24.3% of the population).

We need to achieve group immunity to move freely, make up for lost time, and embrace our loved ones again. Meanwhile, we will have to continue complying with health measures and learn from what we have experienced.

** This article has previously been published on The Conversation by Victoria Sánchez Hellín, Facultative Specialist of Clinical Microbiology. Elche Health Department – General Hospital, Fisabio. Read the original article here.

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