Hong Kong's Summer Flu Epidemic Continues To Decline

HK Flu Express - Week 32

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Hong Kong's unexpectedly heavy summer flu epidemic continues to recede after causing more than 390 deaths since the outbreak was declared on May 5th.  Primarily driven by seasonal H3N2, Hong Kong's summer flu has claimed almost 10 times more lives than did their relatively mild 2017 winter flu season (n=41).

The reasons behind Hong Kong's unusually severe `second flu season' are unclear, although it has been speculated that waning late season flu vaccine effectiveness - particularly among the elderly - may have been a factor. 

Another theory - advanced by two Hong Kong University infectious disease experts (Yuen Kwok-yung and David Christopher Lung) - suggested a minor mutation in the H3N2 virus might be responsible for this summer's unusual epidemic (translation follows):

(1) viral factors: 2014/15-year-H3N2 influenza A winter peak was due to the emergence of antigenic drift (antigenic drift), but the epidemic of H3N2 influenza is not to drift. According to the recent Danish study showed that 2016/17 H3N2 influenza in a small antigenic variation (N121K), this variation has the potential to influenza vaccine used in the past two years failed.

Hong Kong H3N2 influenza A strains also appeared N121K variation, from July 2016 to March 2017, about 20% of the H3N2 strain appears N121K variation, while in May 2017, more than 35 percent of H3N2 strains N121K variation occurs. This variation may be the peak of the flu this summer one of the reasons.

This N121K variation has previously been linked to diminished vaccine effectiveness with the 3C.2a1 clade of the H3N2 virus. You'll find a more complete review of H3N2's recent  evolution in The Enigmatic, Problematic H3N2 Influenza Virus.

First some excerpts from this Week's Flu Express, then I'll return with a postscript.

FLU EXPRESS
 
Flu Express is a weekly report produced by the Respiratory Disease Office of the Centre for Health Protection. It monitors and summarizes the latest local and global influenza activities.

Local Situation of Influenza Activity (as of Aug 17, 2017)
Reporting period: Aug 6 – 12, 2017 (Week 32)

• The latest surveillance data showed that the local influenza activity has further decreased from the peak but some indicators have not yet returned to the levels recorded during inter-season periods. The public should continue to be vigilant.
• The Centre for Health Protection (CHP) has collaborated with the Hospital Authority (HA) and private hospitals to reactivate the enhanced surveillance for severe seasonal influenza cases (i.e. influenza-associated admissions to intensive care unit or deaths) among patients aged 18 or above since May 5, 2017. As of Aug 16, 536 severe cases (including 388 deaths) were recorded. Separately, 19 cases of severe paediatric influenza-associated complication/death (including three deaths) (aged below 18 years) were recorded in the same period.
• Apart from adopting personal, hand and environmental hygiene practices against respiratory illnesses, those members of the public who have not received influenza vaccine may get the vaccination as soon as possible for personal protection.
  

(SNIP)

Surveillance of severe influenza cases

(Note: The data reported are provisional figures and subject to further revision)
Since the activation of the enhanced surveillance for severe influenza infection on May 5, 2017, a total of 555 severe cases (including 391 deaths) were recorded cumulatively (as of Aug 16) (Figure 9). These included:

• 536 cases (including 388 deaths) among adult patients aged 18 years or above. Among them, 486 patients had infection with influenza A(H3N2), 23 patients with influenza A(H1N1)pdm09, 15 patients with influenza B and 12 patients with influenza A pending subtype. 204 (38.1%) were known to have received the influenza vaccine for the 2016/17 season. Among the 388 fatal cases, 176 (45.4%) were known to have received the influenza vaccine. In the winter season in early 2017, 66 adult severe cases (including 41 deaths) were filed.
• 19 cases (including three deaths) of severe paediatric influenza-associated complication/ death. Seventeen (89.5%) cases did not receive the influenza vaccine for the 2016/17 season. To date in 2017, 27 paediatric cases (including four deaths) were filed.

Enhanced surveillance for severe seasonal influenza (Aged 18 years or above)

• In week 32, 65 cases of influenza associated ICU admission/death were recorded (including 51 deaths), which was lower than 57 cases (including 41 deaths) recorded in week 31. In the first 4 days of week 33 (Aug 13 to 16, 2017), 19 cases of influenza associated ICU admission/death were recorded, in which 16 of them were fatal.

Surveillance of severe paediatric influenza-associated complication/death (Aged below 18 years)

• In week 32 and the first 4 days of week 33 (Aug 13 to 16, 2017), there were no cases of severe paediatric influenza-associated complication/ death.

(Continue . . .)

This report also updates us on declining influenza activity in Southern China, Macao, and Taiwan - all of which have been elevated this summer.

• The 2016/17 winter influenza season in the United States, Canada, the United Kingdom and Europe had ended and the influenza activity in these areas remained at low levels.
• In Southern China (week ending Aug 6, 2017), influenza activity was at the summer peak but had started to decrease. The proportion of ILI cases in emergency and outpatient departments reported by sentinel hospitals was 3.6%, lower than that reported in the previous week (4.0%) and the corresponding period in 2015 (3.8%), but higher than that in the corresponding period in 2014 and 2016 (3.4%, 2.8%). The proportion of influenza detections was 22.0%, slightly lower than 24.1% recorded in the previous week. The predominant circulating subtype was Influenza A (H3N2).
• In Macau (week ending Aug 5, 2017), the influenza situation has eased. Recently the number of ILI cases in emergency departments of hospitals has significantly decreased from the peak. The proportion of influenza detections in the last week was 37.3%, and influenza A(H3) constituted 98.2% of the influenza detection.
• In Taiwan (week ending Aug 12, 2017), the numbers and proportions of ILI cases in emergency and outpatient departments showed a decreasing trend. The predominating viruses were influenza A(H3N2), and influenza B constituted approximately 14% of the influenza detection in the week ending Aug 12.
• In New Zealand (week ending Aug 6, 2017), influenza-like illness consultation rates decreased compared to the previous week, while remaining above the seasonal threshold level. The overall influenza positivity rate of tested samples has remained at a high level (about 45%). Influenza A(H3N2) are the predominant viruses in New Zealand this year

Not mentioned in this week's report is the flu situation in Australia, which continues to grow  in intensity. You'll find details on their influenza season in last Tuesday's blog  Australia's Flu Epidemic Worsens.