A recent report from the Health Protection Surveillance Centre (HPSC) attempts to identify the factors underlying the surge in cases of severe disease as well as increased mortality in the past year and whether this pattern is likely to re-emerge in the coming months.

Between October 2022 and August 2023, Ireland experienced a concerning upsurge in cases of iGAS particularly in children under 18 years. Similar increases occurred in other European countries including the UK, Denmark, France and the Netherlands.

In the years prior to the covid-19 pandemic, children accounted for about a quarter (25%) of all iGAS cases in Ireland. Since the 2022-2023 upsurge, the proportion that are children rose to 42%.

Typically, iGAS infections peak in the first six months of the year. The peak that occurred between October and December 2022 was the first hint that something unusual was going on. The HPSC suggests that part of the explanation may be that the covid-19 pandemic interrupted normal patterns of socialisation and disease transmission.

Since October of last year there have been 33 deaths from iGAS—12 in children under 18 years and 21 in adults. Of the 12 deaths in children, 10 children were under 10 years old.

Almost 500 (485) cases of iGAS occurred between January and the end of November this year. This number is a huge increase on the numbers in the previous year (63) but also much higher than pre-pandemic numbers that ranged from 93 to 123 cases a year between 2017-2019.

So what has caused this huge upsurge in numbers for the most severe form of this disease?

One factor is the case definition used. The definition was widened in 2023 to ensure that no cases were missed by routine surveillance. But this widened definition only explains about a fifth (20%) of the increase in cases. Something else other than the way the disease is defined was also going on during this time.

While numbers had returned to expected levels between August and October this year, worryingly a slight increase has occurred in adult cases since November this year. It is too early to predict the trajectory over this coming winter but HPSC will continue to closely monitor these numbers over the coming months.

What caused the high levels of severe disease with iGAS in the past year?

A number of factors are linked to the high number of cases of severe and unfortunately fatal cases of iGAS over the last 14 months.

Data analysis shows evidence of circulation of particular subtypes or emm-types during the last year; with emm1 predominating in over half (56%) of cases followed by emm12 (23%) and emm28 (4%). This pattern is unusual. In pre-pandemic years emm1 would have accounted for only 14% of cases with emm12 (11%) and emm28 (7%).

The upsurge that was identified in the UK in November 2022 was followed shortly afterwards by the identification of a similar increase in Ireland. This increase was characterised by low iGAS activity during the pandemic followed by a rise in respiratory virus activity that began towards the end of 2022.

Rising levels of respiratory infection included increases in influenza, covid-19, respiratory syncytial virus (RSV) and also bacteria such as invasive pneumococcal disease and Haemophilus Influenzae, as society began to open up with the lifting of restrictions; and with it the emergence of sub-types of iGAS (emm1 and emm12) that are known to be associated with more severe disease and poorer outcomes.

Also noted was an increase in hospitalised chicken pox (varicella) especially among children (0-17 years). There is a firmly established relationship between varicella infection and subsequent development of iGAS—numerous studies report that iGAS infection is preceded by varicella in about a fifth (20%) of cases. 

There is no such clear relationship in adults. In this current upsurge there was a clear association between varicella and infection with iGAS in children. Almost a fifth (17%) of children were co-infected with varicella and iGAS while almost a quarter of children (23%) with iGAS reported varicella as a risk factor.

This increase in varicella drove, in part, the iGAS surge in the first half of 2023.

Pandemic measures to control covid-19 

During the pandemic, measures to control the spread of covid-19 included closures of schools and workplaces with the advice to work at home where possible; social distancing; the wearing of masks; importance of good hand hygiene as well as advice not to travel. All of these measures also led to low levels of other infections including iGAS.

"Reduced exposure to iGAS during the pandemic is likely to have contributed to increased susceptibility to infection in the general population, with particularly severe infection in the most vulnerable subjects," according to the HPSC report.

"As there is a close association between iGAS and winter respiratory viruses, close monitoring and scrutiny of iGAS activity is continuing during the winter."

Symptoms of iGAS 

It’s not always easy to tell if you have iGAS. Symptoms can appear like symptoms of other conditions. The type of symptoms to watch out for include: high temperature (38C or higher), muscle tenderness or severe muscle aches, increasing pain, redness and swelling at the site of a wound; dizziness and confusion; a flat red rash over large areas of the body; unexplained diarrhoea or vomiting.

Having these symptoms does not always mean you have iGAS but does mean you should seek urgent medical help if you or your child feel unwell, particularly if you have had recent exposure to a respiratory illness or chicken pox.

Trust your instincts and remember that one of the best ways to curb circulating respiratory viruses, apart from keeping up to date with flu and covid-19 vaccines is to wash your hands thoroughly with soap and water and keep away from others when you or your children feel unwell.

• Dr Catherine Conlon is a public health doctor in Cork and former director of human health and nutrition, safefood