The key is in the activity of the immune system and in the proliferation capacity of the virus in these people. A trained immune system clears the virus before it can cause serious damage to the body.
Therefore, the question to be cleared now is whether immunized people maintain contagion capacity. The answer will depend on the amount of virus they can spread.
Infections after immunization: less severe or asymptomatic symptoms
A recent study carried out in senior centers in Chicago showed that 4.2%, among workers and patients, were infected with SARS-CoV-2 in a period of four months (December 2020-March 2021). Of those infected, most had not been vaccinated. But 6% of the infections had occurred in fully vaccinated people and 23% in those who had received a single dose.
On the other hand, reinfections of people who have had the disease are rare but do occur. In a study conducted in the United Kingdom with healthcare workers who had suffered COVID-19, 0.6% suffered reinfection. Of course, with mild symptoms.
In addition, the recent SIREN study focused on immunity and reinfection by SARS-CoV-2 concludes that the immune response largely prevents the risk of contagion. But it also indicates that, even with mild symptoms, those reinfected can be a source of spread of the virus. In the United States, exposure to the new variants has been considered as a factor against possible reinfection in already immunized people.
As for Spain, Cases of fully vaccinated people who have been reinfected with mild symptoms accompanied by high viral loads have already been reported. The same has happened in other countries such as Singapore or the Seychelles.
Antibodies are not everything: the relevant role of T lymphocytes
Throughout the pandemic, much attention has been paid to the levels of antibodies and how long they remain in our blood. But are antibodies really that relevant?
Antibodies are produced by activated B lymphocytes that transform into plasma cells. Plasma cells stop working over time and die. The antibodies produced by these remain circulating in the blood for weeks or months until they degenerate and are eliminated.
Memory B lymphocytes are also produced that will act more quickly and generate plasma cells and antibodies more quickly and efficiently in the event of reinfections. Therefore, the duration of the antibodies in the plasma is not so important: it is the memory cells that matter.
T helper lymphocytes are also activated in the immunization process. These are cells responsible for controlling the activity of the immune system. Simultaneously, cytotoxic T lymphocytes come into play, which act against the cells that are expressing the virus protein and eliminate them, thus reducing the proliferation of the virus. In both types of lymphocytes, memory cells are generated with the ability to rapidly activate in reinfections.
T lymphocytes are also essential against virus infections by releasing interferon, a signaling protein that blocks virus replication.
The SARS-CoV-2 mutations are especially affecting protein S. Those variants that present a change in this protein that improves its binding capacity to human protein, increase its infective capacity. For this reason, the variants that are being imposed around the world are presenting similar changes.
Does it mean that vaccines lose effectiveness? The vaccines generate antibodies against different areas of the protein S. They also activate T helper and cytotoxic lymphocytes that act by recognizing different areas of the protein. Therefore, the point mutations that are taking place in the protein S of the virus do not have to affect the immune response in a significant way.
Immunization only protects the immunized person
The vaccine does not totally prevent the virus from invading. Both antibodies and lymphocytes “prepared to defend ourselves” are found within our body. The virus infects us mainly through aerosols in the air, so it begins by invading the cells that line the upper part of the respiratory system.
This implies a relationship between the ability to infect and the reaction time of the immune system. Immunized people have a trained system that will act in a short time. But in the meantime, the virus can proliferate and the person would be infected but without symptoms. And you can infect others.
The simple fact that the virus is opposed by the immune system almost immediately suggests that there will be a reduction in its transmission capacity. In fact, a recent study in the UK (without peer review) indicates that up to 50% less.
Since immunization increases the number of asymptomatic people who do not know they are infected, we cannot lower our guard. For this reason, the European Center for Disease Prevention and Control (ECDC) warns about infections from immunized people.
It is essential that group immunity is achieved to reduce the spread of the virus and infections.