Getting Covid after being vaccinated can happen — but it’s rare

In late July, the United States (US) reversed the mask-wearing rules for people who were fully vaccinated against Covid stipulating that they’d have to again wear masks in indoor public places. 

This overturned a May decision that vaccinated people could mostly stop wearing masks in public because they were well protected against infection and were unlikely to spread the virus to others. 

To reduce the risk of being infected with the highly transmissible Delta variant and possibly spreading it to others, the Centres for Disease Control (CDC) now says, vaccinated people need to wear masks in indoor public spaces, especially if they live in areas of “substantial or high transmission”.

Moreover, the CDC has instructed all teachers, staff and students to wear masks, regardless of whether they have been vaccinated or not. 

The US has fully vaccinated just over half of its population.   

So why the turnabout in rules? 

Breakthrough infections among people who were fully vaccinated.

Barnstable county in Massachusetts has vaccinated almost 10,000 people (which is around 5% of the town’s population). July surveillance from the CDC found 469 Covid cases in the town — almost three-quarters of which were in people who had been fully immunised. This is equivalent to a breakthrough rate of around 3.6%. Among those who were infected, 90% of cases had been caused by the Delta variant, which is now also dominant in South Africa.

Data from the Washington health department found 4,241 infections in fully vaccinated people during a six-month period ending in July. At the end of that month, the state had administered 8,179,883 vaccines.

And the CDC compiled data from 49 out of America’s 50 states: of the 163-million Americans who had been fully vaccinated by July, 6,587 people had been hospitalised or died as a result of breakthrough infections.

The reason? 

At a press briefing, the CDC’s director, Rochelle Walensky, explained: “The Delta variant is showing every day its willingness to outsmart us.”

What are breakthrough infections?

A breakthrough infection is when a person gets sick despite being vaccinated for that illness. In the case of Covid, that would be when someone who has been fully vaccinated gets infected with the SARS-CoV-2 virus and still develops Covid (with or without symptoms). 

Countries such as the US have been collecting real-life data on their vaccine roll-outs and have begun seeing SARS-CoV-2 breakthroughs as a result of infection with the Delta variant. Delta was first detected in India in October and is significantly more transmissible than any other form of the virus.

As a result, the US increased the measures that fully vaccinated people need to take to protect themselves, and those around them, against SARS-CoV-2 infection. So for now, mask-wearing in public spaces for vaccinated people is on again — at least indoors. 

But does that mean Covid jabs aren’t working?

Not quite.

As the US data on breakthrough infections show, only a small proportion of people who are fully vaccinated get infected with SARS-CoV-2, and the infections tend to be mild.

The infections are generally mild, because what Covid vaccines are best at doing is protecting us from falling severely ill — or dying — if we develop Covid-19.

Data from England, for instance, shows that just one shot of the Pfizer jab (we use this vaccine in South Africa and you need two doses to be fully vaccinated) reduces your chances of hospitalisation as a result of infection with the Delta variant by 94%. Two shots increase the protection to 96%.

But the protection offered by the same vaccine against developing milder forms of Covid caused by the Delta variant is lower than the protection it offers against falling severely ill. 

England data shows one jab offers 36% protection against such infection and two shots reduce your chances of infection by 88%.

And Israel reported a 10% breakthrough rate among healthcare workers who had been fully immunised with the Pfizer vaccine, according to a July study in the New England Journal of Medicine. Most of these infections were caused by the Alpha variant, which was first identified in England and was dominant in Israel at the time. Among the breakthroughs, two-thirds were mild Covid cases and the remaining third did not develop any symptoms.  ​

Getting vaccinated against Covid, therefore, doesn’t necessarily mean that you’re done with the disease, because there’s still a chance that you can get infected. But what it does mean is that your chances of ending up in an intensive care unit or dying of Covid are drastically lower than someone who hasn’t been vaccinated. 

But it’s important to remember: No Covid jab is 100% effective. 

What is vaccine efficacy?

So what does it mean if the Pfizer jab provides 96% protection against hospitalisation? 

What it doesn’t mean, is that 96% of people who had been fully vaccinated with the vaccine wouldn’t end up in hospital. Rather, it means that each person who gets fully vaccinated reduces their own risk of hospitalisation as a result of infection with the Delta variant by 95%. 

Some people will, therefore, still fall seriously ill as a result of Covid because the protection isn’t 100%.

But there’s also something called general vaccine efficacy of Covid vaccines — and that refers to someone’s risk of developing Covid after they’ve been immunised. 

Each Covid vaccine has its own efficacy. So the efficacy of Pfizer’s jab which offers 88% protection against developing Covid caused by Delta will be different from the efficacy of the AstraZeneca vaccine which is 67% effective against disease caused by the Delta variant. 

Real-life data (such as the data from England that tells us how well the Pfizer vaccine works against the Delta variant) is different from trial data. 

Trial results only tell us how much protection a vaccine can offer in an ideal setting because studies are conducted in tightly controlled settings, which are different from real life. In trials, all vaccinators, for instance, receive exactly the same training, and study participants all return for the second shot of a two-dose vaccine precisely the same number of days after their first dose. But in real life, things may not always play out that way.  

And when a virus has been around for a while, it adapts for survival and develops mutations which can make it more transmissible or help it to escape immunity (by making the antibodies we produce as a result of vaccines work less well) — that also affects how well a jab will work in real-life. 

So although Pfizer had an original efficacy of 95% in its trial (the results were shared in November 2020, before the emergence of variants) this was lower when the vaccine was rolled out. Real-life data from the US, for example, shows that the vaccine had an effectiveness of 86% in adults (and not 95%) in April 2021.  

Importantly, we don’t know if Covid vaccines actually prevent you from getting infected. This is because most trials were designed to detect symptomatic cases. In other words, the results from these trials are less focused on whether people are being infected with the SARS-CoV-2 virus and rather determine if people are developing the disease — Covid-19 — which the virus causes. 

Because the trials are designed to pick up people’s symptoms, like a cough or fever, they could miss asymptomatic cases, where the person themselves doesn’t know they are sick.

A January paper published in JAMA estimates 30% of people with Covid don’t develop any symptoms, yet they are still infectious. The projections show that 59% of new Covid cases were caused by people who had no symptoms of the disease.

Why do breakthrough infections happen? 

• New variants can reduce the efficacy of vaccines

• Logistical reasons