The mysterious vanishing of MIS-C

The hyperinflammatory syndrome after Covid-19 in children has all but disappeared

Whilst the overwhelming majority of Covid-19 infections in healthy children are mild or asymptomatic, the serious risk which remained was of the hyperinflammatory syndrome known as MIS-C (Multi-system Inflammatory Syndrome associated with Covid-19). This condition occurs around 2-6 weeks following infection, and a high number of children required treatment in intensive care. Symptoms included persistent and prolonged fever, abdominal pain, often a rash, and the child appearing clinically very unwell.

Following the UK vaccination campaign and reopening of much of society in summer 2021, cases of Covid-19 amongst all age groups increased. The main fear for children was that we would see a large spike in cases of MIS-C.

Mysteriously, that never happened.

Even more mysteriously, the condition appears now to have all but vanished.

The following chart depicts 2 things:

In blue is the fortnightly prevalence of infection in children aged 2 - 17, according to the Office for National Statistics infection survey (this tests thousands of people in the UK regardless of symptoms in order to get an accurate estimate of exactly how many people are infected at any one time, so is not influenced by how much the public are getting tested for Covid-19).

In red is the number of children being admitted to paediatric intensive care units (PICU) with MIS-C, according to PICANET (this is the national audit data for PICUs across the country).

The data is normalised to the peak in January 2021. For a sense of scale, the absolute numbers at this stage were 34 PICU admissions per week for MIS-C, and a Covid-19 prevalence of 3.11%).

MIS-C PICU admissions are offset by 21 days to more closely match the prevalence trend.

As you can see, during the Delta wave in England the rate of MIS-C cases never matched the same proportion or total number of cases as occurred during the Alpha wave, despite much higher rates of Covid-19. This was a huge relief, although a bit of a surprise to paediatricians at the time.

Even more mysteriously, since Omicron has arrived rates of MIS-C have all but vanished. The most recent PICU admission numbers from May 2022 were at 1 per week - across the whole country!

This is obviously very welcome news! This was the most feared consequence of Covid-19 infection by paediatricians. But it does leave us asking questions…

Why has MIS-C vanished?

There are 2 main theories as to why MIS-C seems to have almost disappeared:

The virus has changed

We are still not clear on precisely what leads to children developing MIS-C, but we do know the SARS-CoV-2 virus has mutated significantly from the original variants which caused higher rates of this condition. It could be that changes to the spike protein render the virus less likely to trigger MIS-C, both for Delta and even more so for Omicron.

The children have changed

The overwhelming majority of children in the UK have now been infected with SARS-CoV-2 at least once, and therefore have existing immunity. We know that vaccination seems to significantly reduce the risk of MIS-C, which suggests that existing immunity is protective. It could be that since everyone has now been infected, the subsequently occurring reinfections are simply much less likely to lead to MIS-C.

It may even be a combination of both of these factors. In either case, paediatricians everywhere will be celebrating the mysterious vanishing of this condition.

Here’s hoping it remains that way for a long time.