The development of several vaccines against Covid-19 in less than a year has given us all hope of an exit from the pandemic. The goal is now to ensure extensive immunization coverage as quickly as possible, all over the planet.
However, vaccines are not 100% effective in stopping transmission or infection. The risk that some fully vaccinated people will be infected is small, but it is to be expected that this will happen.
It is important to be aware of the limitations of vaccines. No vaccine offers total protection to everyone who receives it. The measles vaccine has been shown to be very effective in preventing infection, which has led to virtual eradication of the virus in some countries.
Yet infections are reported even in widely vaccinated populations. These infections don’t just happen in unvaccinated people; there are cases of infections in fully vaccinated people.
The seasonal flu vaccine provides protection against circulating viruses. But circulating influenza viruses vary, and people who are vaccinated can still get sick, but less seriously.
This phenomenon is explained by the fact that immune defense occurs in different ways. On the one hand, there are the antibodies, which are Y-shaped proteins that lock onto germs and neutralize them, and on the other hand, the T lymphocytes, which find and destroy infected cells.
Antibodies are usually directed against the more variable proteins on the surface of the virus, while the more constant proteins – inside the virus – are targeted by T lymphocytes. T lymphocytes are important in decreasing the severity of the disease. .
Regarding SARS-CoV-2 (the virus responsible for Covid-19), there are a few cases of post-vaccination infection in Seychelles, but little information has so far been published in scientific journals.
A recent report in the New England Journal of Medicine describes two cases of Covid-19 after vaccination. Both people had mild symptoms which disappeared within a week.
And an study from Stanford University, which has yet to be evaluated by other scientists, describes 189 post-vaccination Covid-19 cases in 22,729 healthcare workers. However, some of the infected workers had received only one dose of the vaccine. Vaccination will likely make the disease less severe if such outbreaks occur.
Several explanations
There may be several reasons for post-vaccination infections. The immune response in humans is encoded in our DNA and varies from person to person. This variability helps us respond to a wide variety of viruses. But the effectiveness of these responses is also variable. It depends on several factors, including health, medication or age.
The aging immune system does not respond to new antigens (a foreign substance that causes your immune system to make antibodies) and vaccines as well as younger immune systems. For a vaccine against Covid, there was a measurable difference in the concentration of neutralizing antibodies in the elderly compared to younger adults. Some elderly participants had no neutralizing antibodies at all after receiving both doses of the vaccine.
Post-vaccination infections can also be caused by variants of the virus that escape immune detection and even develop in people who have been vaccinated. A virus, especially an “RNA virus” like SARS-CoV-2, is expected to mutate and give rise to variants, some of which may be more contagious. These variants are neutralized more or less efficiently by the immune system, because mutations can modify the parts of the virus recognized by antibodies and T lymphocytes.
A new variant of SARS-CoV-2 identified in India (B.1.617.2) would make the virus more contagious, which is worrying given the ongoing health crisis in this country. Despite the lack of scientific studies, the media report numerous cases of frequent infectious breakthroughs and the B.1.617.2 variant is implicated, but this remains to be proven.
Read more: Indian variant: is it more contagious, more dangerous?
A study carried out on post-vaccine SARS-CoV-2 infections in California showed that risks of infection were not higher due to the variants in this region.
Despite the evidence that vaccines work well against variants, the rapid increase in the proportion of cases in the UK which is attributable to the B.1.617.2 variant compared to the dominant Kent strain (B.1.1.7) has elevated this mutation to the rank of worrying variant by Public Health.
gajendra bhati / Alamy Stock Photo
While widespread vaccination remains the ultimate solution in a pandemic, it should be mentioned that it is unlikely to prevent all infections. However, people who catch the Covid after being vaccinated will likely have milder symptoms. The risk of post-vaccination infections should therefore not deter us from getting vaccinated. Further studies into the causes of SARS-CoV-2 infections could help scientists improve Covid-19 vaccines or change the timing of boosters.
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