There are a large number of news reports on the flu season in USA currently, including this fairly comprehensive one from CNN, after the statistics for  mortality from pneumonia and influenza in the USA crossed 6.8% (the epidemic threshold) on Week 51 of 2014. There have been 15 paediatric deaths to date (as of 20th December 2014). Below are two slides from the weekly influenza statistics provided by CDC Atlanta, showing how the 2014-2015 influenza season compares to previous years (already higher number of outpatient cases at this early stage), and the epidemic as well as seasonal baseline thresholds for influenza, which looks like a sinusoidal wave.

Percentage of outpatient visits for influenza-like illness (ILI), captured by the US Outpatient ILI Surveillance Network (Source: CDC Flu Weekly).

Seasonal baseline and epidemic thresholds for influenza and pneumonia mortality in the US (Source: CDC Flu Weekly).

The vast majority of influenza cases appear to be due to influenza A(H3N2) this time, rather than the former pandemic influenza A(H1N1).

Influenza subtypes – mainly H3N2 influenza A in this current influenza season in the US (Source: CDC Flu Weekly).

What makes this influenza season particularly challenging is that the current influenza vaccine is less effective against this time round. Among the circulating influenza A(H3N2) viruses in the US currently, only 32.6% belonged to the strain that is included as a component of the Northern Hemisphere 2014-2015 influenza vaccine, whereas the remainder were different, which means that even a mass vaccination campaign is unlikely to be helpful to mitigate the impact of the epidemic. This mismatch arose because the mutated H3N2 virus only started to surface and spread in March 2014, after vaccine production had started.

What about antiviral resistance? It appears that the current circulating viruses in the US are highly resistant to amantidine and rimantidine (not commonly prescribed in Singapore, or anywhere else), but none of the viruses tested were found to be resistant to the neuraminidase inhibitors (i.e. oseltamivir or tamiflu, zanamivir, and peramivir). Is this relevant?

Expert opinion seems to be mixed in this area of the use of neuraminidase inhibitors for the treatment of influenza. Members of the Cochrane Collaboration – a highly respected global network of independent researchers that attempt to make sense of the flood of clinical and experimental biomedical data using systematic reviews and meta-analyses – recently reviewed all the clinical trial data and pharmaceutical company internal reports of oseltamivir (Tamiflu) and zanamivir (Relenza), and concluded that there was no good evidence to suggest that these drugs could reduce hospital admissions or complications from influenza, although the symptoms of influenza could be shortened by half a day. When used for preventing influenza transmission, the Cochrane group found that oseltamivir could reduce the risk of people suffering from influenza, but there was a real increased risk of people suffering from headaches, psychiatric or renal (kidney) adverse events (as is also the case when these drugs are used for treatment). The experts at CDC Atlanta, on the hand, continued to advocate the use of the neuraminidase inhibitors for the treatment or prevention of influenza. Their own study (unfortunately behind a pay-wall) – performed in Bangladesh in collaboration with the International Centre for Diarrhoeal Disease Research Bangladesh (ICDDR,B) – was published last year in Lancet Infectious Diseases, showing a modest reduction in symptom duration and virus shedding.

What does it mean for us in Singapore? It is probable that the vaccine mismatch problem will similarly surface in Singapore, given the propensity for influenza viruses to spread around the globe. It does not mean that influenza vaccination should be postponed until the Southern Hemisphere vaccine is available on the market, however, as the current influenza vaccine may still be protective (against the other circulating influenza viruses). Those children and adults who fit the list of criteria published by the CDC should also opt to be vaccinated with the pneumococcal vaccine. There is an unfortunate association between influenza-like illnesses and bacterial pneumonia, and post-influenza bacterial pneumonia – usually caused by the pneumococcus (Streptococcus pneumoniae) and Staphylococcus aureus – is associated with high morbidity and mortality.