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Thursday 1 June 2023 Dublin: 15°C
# breakthrough cases
Explainer: Here's why a limited number of Covid-19 cases in vaccinated people is to be expected
The vaccines aren’t 100% effective, but they are a significant barrier against severe illness from Covid-19.

IN RECENT DAYS, cases of Covid-19 among those who are fully vaccinated may have caused concern, particularly among those who are not yet vaccinated.

When the public was urged to get vaccinated because they offer ‘protection’ against Covid-19, does that not mean protection from testing positive for it?

No, is the short answer. People who are fully vaccinated can test positive for Covid-19, but that doesn’t mean vaccines don’t work. They’re still doing the job they are meant to do.

The explanation is simpler than it might first seem. 

The best way to explain it: seatbelts

If you don’t have a lot of time for a load of figures, percentages, and nuanced public-health rationale (and after the last 18 months, that is understandable) – here is the best way to explain breakthrough Covid-19 cases in fully vaccinated people.

Around a quarter of people who die on our roads were not wearing a seatbelt, according to the Road Safety Authority. That means in 75% of deaths on our roads, the person who died was wearing a seatbelt. 

This doesn’t mean that seatbelts don’t work, that they’re not worth wearing.

Those figures also don’t show where seatbelts have prevented someone from dying in a car crash – or prevented them from being seriously injured. That is their main aim: to reduce those very serious outcomes for a population as a whole.

Our roads (or getting Covid-19) can be dangerous, and a base level of precaution should be taken by every citizen to keep people safe.

What is the benefit of a vaccine if you can still test positive?

The main aim of the Covid-19 vaccines is to prevent Covid-19 from making people seriously ill, or dying. 

After vaccines began to be rolled out in Israel, the UK, EU and US, there were questions about how they would affect transmission of Covid-19. And while it’s since been shown that vaccines do reduce transmission, the do not completely prevent Covid-19 from being passed from one person to another.

Deputy Chief Medical Officer Dr Ronan Glynn said on RTÉ’s News at One on Thursday: “No vaccine is 100% effective, so there will be cases in people who are fully vaccinated.

“But the key element of this is the really fantastic effectiveness of these vaccines against symptomatic disease, and even more importantly, against serious disease that results in hospitalisation, and admission to critical care, or death.”

The vaccines are 80-85% protective against symptomatic disease, Dr Glynn said, and about 95% effective against hospitalisation.

Their main aim is to prevent death, and also serious illness that would mean you need to be hospitalised. And the public health experts are happy that the number of cases we currently have are not corresponding to the influx of hospital admissions there were before in previous waves of Covid-19.

This is what they mean when they say the link between infection and hospitalisation is “weakened” but not broken.

Previously, 50 or more in every 1,000 cases of Covid-19 would require hospital treatment, but it’s now down to 20, or even 10 per every 1,000 cases in Ireland.

This article from the New York Times explains how breakthrough cases can happen among a group of people who are 95% fully vaccinated, but compares this with a group where only 20% are fully vaccinated.

Context is important

Irish health officials have also explained how cases of Covid-19 among fully vaccinated people should be read in context of how that is leading to less severe cases than before.

NPHET member Prof Philip Nolan said on Thursday that with 70% of the population fully vaccinated, they will expect 30% of cases and 15% of hospitalisations because of Covid-19 to be among those who are fully vaccinated.

When we first read that, it can seem jarring.

But it’s important to read the figures on ‘breakthrough cases’ in the context of the figures we have been used to seeing in the past 18 months.

You could infer from that – as per Philip Nolan’s figures – that vaccines are at least 85% effective against preventing hospitalisations in people who catch Covid-19, even if it’s the more transmissible Delta variant.

When we see those numbers, they are worth comparing to the rate of hospitalisation and ICU admission we had prior to high rates of vaccination.

Deputy Chief Medical Officer Dr Ronan Glynn gave this example:

“… While 67% of cases in those who are 65 years and older in the last fortnight have been in people who reported having received two vaccines, the absolute number of cases in this age group (764) is much less than in previous ‘waves’.

A similar disease profile last February, for example, resulted in 3,379 cases over a two-week period in the same age group. 
This suggests that at-risk groups are more than four times less likely to end up in hospital if they are vaccinated.

“What we don’t see is that for every fully vaccinated case, vaccines are preventing about 4 other cases,” Prof Nolan said today. “Vaccines are preventing at least 2,700 cases per week per million population, probably more.”

This graph illustrates the difference between the rate of cases in those who are vaccinated, versus those who are not:


Another factor in why we are seeing cases among fully vaccinated people is because of the now-dominant Delta variant of Covid-19.

The vaccines were highly effective at preventing symptomatic infection from previous variants, such as the original strain of the virus that from the early stages of the pandemic.

The same can be said when it comes to preventing infection from Delta, but the figure isn’t quite as high due to new mutations making this variant better at evading the body’s immune response.

Some numbers

On Monday, Chief Medical Officer Dr Tony Holohan explained that of the 20,000 cases notified over the previous fortnight, 17% were in people who were fully vaccinated. But a mild illness is the norm among that 17%.

Between now and April, the vast majority of Covid-19 ICU admissions and deaths have been among those who were not fully vaccinated.

Dr Ronan Glynn said on Wednesday: “Of 169 adults admitted to ICU with Covid-19 since 1 April, just six had been fully vaccinated more than 14 days prior to their diagnosis.

“Of 155 adults who have died with Covid-19 since 1 April, just seven had been fully vaccinated more than 14 days prior to their diagnosis.”

Dr Glynn said that the HSE cyber attack means we don’t yet have access to information on the fully-vaccinated people who have been admitted to hospital.

As the rate of fully vaccinated people increase above 90%, there will still be people who are hospitalised with Covid-19, because the vaccines are not 100% effective at preventing a level of illness that requires hospital treatment. But the number of people who are hospitalised will be much lower in number than in previous waves.

This is how we end up with figures that show a limited number of vaccinated people being hospitalised or being admitted to ICU with Covid-19.

Prof Nolan said: “If vaccine coverage increases to 90%, the majority of cases, and close to half the hospital admissions will be in vaccinated people, but [there will be] a smaller number of cases and hospitalisations.”

Dr Glynn told RTÉ earlier in the week: “At any other point in the pandemic, with the disease incidence that we have now, with the level of cases that we have now, we had very significant parts of out society shut down.

Because of vaccination we’ve been able to keep the vast majority of society open over the last number of weeks despite increasing incidence.

Although breakthrough cases are inevitable, they are outliers among the vaccinated population, and the majority of hospital admissions with Covid-19 continue to be in those who have not been fully vaccinated. 

With reporting from Nicky Ryan.

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