Understanding Covid-19 vaccines: What you need to know about 'breakthrough infections'

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  • Health officials have touted the importance of getting vaccinated against Covid-19, despite breakthrough infections. 
  • Vaccination does not completely protect you against infection, but if you do get Covid-19, you’re likely to avoid severe illness and death.
  • Infection occurs in just a fraction of vaccinated persons and accounts for a very small percentage of Covid cases. 

Over the last few weeks, reports have emerged of people who became infected with Covid-19 despite being fully vaccinated, causing many people to wonder whether getting inoculated is worthwhile.

But these cases, known as “breakthrough infections”, are to be expected, especially if population immunity levels aren't yet high enough. While the current vaccines are highly protective against Covid, they are, however, not perfect.

South Africa’s National Institute for Communicable Diseases (NICD) explains: “Once population immunity levels rise with increased vaccination, there will be less transmission of the virus and fewer breakthrough infections.”

They encourage any adverse events after vaccination to be reported via the MedSafety app to be investigated.

Breakthrough infections: less severe illness

The important point is that in the small percentage of vaccinated people who go on to test positive for Covid, infections are likely to be mild or asymptomatic (without symptoms). In other words, the majority of vaccinated people will be protected against the most severe effects of the disease, including hospitalisation and death – which is the key purpose of the vaccine. 

Updated information by the US Centers for Disease Control and Prevention (CDC) states that according to large-scale clinical trials the vaccines prevented most people from getting Covid – and that real-world data involving the mRNA vaccines by Pfizer-BioNTech and Moderna reflect these results. 

“While these vaccines are effective, no vaccine prevents illness 100% of the time. For any vaccine, there are breakthrough cases,” the agency says.

An earlier analysis by The Associated Press (AP) found that out of 107 000 US patients who were hospitalised with Covid-19 in May,  98.9% were unvaccinated. And of all Covid-19-related deaths that occurred in May, only about 150 of the more than 18 000 were in fully vaccinated people.

Breakthrough infections can also happen as a result of people catching Covid just after vaccination, as it typically takes around two weeks for vaccine-induced protection to kick in, it explains.

Study of breakthrough cases

A small study of healthcare workers in Israel found that the majority of 39 breakthrough Covid cases among 1 497 participants were mild. Nearly all (91%) of the participants received the two-dose Pfizer vaccine.

The researchers also noted that 85% of these cases resulted from the Alpha variant (first identified in the UK), as their study was not designed to address the issue of variants of concern (VOC) in breakthrough cases.

The findings also include information on secondary infections. The authors said, "No secondary infections were traced back to any of the breakthrough cases, which supports the inference that these workers were less contagious than unvaccinated persons."

Their study, published in The New England Journal of Medicine on 28 July, had a few limitations, including a cohort that represented mostly young and healthy people. Since all breakthrough cases were mild and did not require hospitalisation, the team couldn't determine how well the vaccines protected against severe infection, or infection in older, vulnerable individuals with coexisting illnesses.

Additionally, nearly one in five vaccinated participants who had breakthrough infections reported suffering from long Covid, where they experienced symptoms more than six weeks after their infection had cleared. 

Factors linked to breakthrough cases

There are also certain factors that make breakthrough infections more likely to happen, Dr Sanjay Mishra, Project Coordinator & Staff Scientist at Vanderbilt University Medical Center in Tennessee, USA, explained in an article for The Conversation

These include being in situations of close contact with others, such as in cramped working environments, restaurants, parties, or a stadium. Breakthrough infections, he added, are also more likely among healthcare workers since they are often in contact with Covid patients.

Another factor is age: the vaccines have shown to trigger a less robust immune response in older people, causing the risk of a breakthrough infection to be higher with increasing age. 

Being immunocompromised or having underlying health conditions such as high blood pressure, diabetes, heart disease, and cancer also puts one at higher risk of a breakthrough infection that can lead to severe Covid, Mishra added.

Another reason to get vaccinated

Health officials have warned that unvaccinated individuals will give the SARS-CoV-2 virus (which causes Covid disease) the opportunity to not only spread but mutate, which could then lead to newer, highly infectious, and even deadlier variants, Health24 recently reported.

The Delta variant, for instance, is considered the fittest VOC to date and has already spread to more than 100 countries. It is currently dominating infections in South Africa.

Although the virus can only be successful at replicating by infecting the body of a human, and vaccinated individuals can still catch Covid, the chances of this happening in the body of someone who is vaccinated is much lower than in someone who is not vaccinated.

‘Moot argument’

"Even complete vaccination (and two or more weeks after the last dose by which time immunity has developed fully) is not 100% protective against infection. But this is a moot argument," Professor Wolfgang Preiser, head of the Division of Medical Virology at Stellenbosch University, told Health24.

"The odds of infection [in vaccinated individuals] are hugely reduced – the chances of becoming infected are much lower for the vaccinated compared to the unvaccinated.  

"In addition, vaccinees who still become infected are likely to have milder disease and lower viral loads, meaning there is less viral replication happening and thus a reduced chance of mutations occurring; plus they are probably less likely to pass it on," he said.

"The risk will not be zero, but ... it is disingenuous to use the small remaining risk to argue against the principle,” added Preiser.

*For more Covid-19 research, science, and news, click here. You can also sign up for our Daily Dose newsletter here.

READ | Covid-19 vaccine myths debunked: Get the facts here

READ | Latest study shows two shots of Pfizer vaccine effective vs Delta - but getting second shot vital

READ | App allows users to report adverse effects from medicines, including Covid-19 vaccines

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