Young and old most susceptible in face of rising flu burden

Influenza burden in Singapore has been on the rise since 2010, with young children (aged ≤4 years) and the elderly (aged ≥65 years) being most susceptible to influenza‐associated hospitalization for pneumonia and influenza (P&I), a local study has shown.

“Influenza vaccination programmes should continue to be prioritized for the young and the elderly,” said the researchers led by Ng Yixiang, a statistician at the Epidemiology and Disease Control Division, Ministry of Health, Singapore. 

Rising burden

From 2010–2017, influenza contributed to an estimated 16.3 percent of all P&I hospitalizations in Singapore, corresponding to an excess rate of 50.1 per 100,000 person‐years for influenza‐associated P&I hospitalization. [Influenza Other Respir Viruses 2019;doi:10.1111/irv.12676]

The estimated rates are considerably higher than the previous estimates of 11.2 percent (29.6 per 100,000 person-years) using data from 2010–2012.

According to the researchers, the greater burden observed in the current study was due to the higher estimated rates in 2016 and 2017, when influenza A(H3N2) was dominant. The annual influenza positivity percentage ranged from a low of 40.9 percent in 2011 to a high of 53.6 percent in 2016, with the positivity rates consistently exceeding 40 percent every year.

“2016 was the first year since 2010 that our local influenza positivity data exceeded 50 percent, suggesting a resurgence in virus transmissibility since the post‐influenza A(H1N1) 2009 pandemic period,” observed Ng and co-authors.

“The higher influenza positivity percentages in 2016 and 2017, together with the surge in observed P&I hospitalization rates, would result in larger estimates for hospitalizations attributable to influenza in Singapore in these 2 years,” they explained. “The increase in observed P&I hospitalizations might be due to the varying composition of respiratory viruses circulating in the community, but it could also be due to an increased susceptibility from an ageing population.” 

Susceptible subgroups

In particular, the estimated excess rates for influenza‐associated P&I hospitalizations were higher for the youngest (0–4 years; 186.8 per 100,000 person‐years) and the oldest (≥ 65 years; 338.0 per 100 000 person‐years) age groups.

Also, influenza burden was higher during the middle of the year (from April to September) compared with the end-to-start period of the year (from October to March; mean weekly P&I hospitalizations associated with influenza, 55.6 vs 48.3; p=0.006).

“Vaccination has been found to be one of the most effective ways to reduce influenza burden and also a cost‐effective intervention strategy for targeted age groups,” noted Ng and co-authors. “The higher rates estimated for these two groups suggest that influenza vaccination programmes should continue to be prioritized for the young aged 0–4 years and the elderly aged 65 years old and above.”

“[The peak in mid-year] shows that we may also have to vaccinate during the middle of the year, in addition to the recommendation by a WHO study to vaccinate in October,” they said, calling for further studies to be done to work out the optimal vaccination schedule in places with year-round influenza activity.

As the current study only took into account of P&I hospitalizations, which represent a subset of respiratory or all-cause hospitalizations, the true influenza burden could have been underestimated.