One of the lessons of the past 15 years is: just when we think we've got novel flu viruses figured out, they do the unexpected. And this winter is no exception.
After last winter's record number of human H7N9 infections in China (see chart above), just about everybody expected a repeat performance. Yet, since October 1st, only two cases have been reported by Mainland China.
Why? No one really knows, although there are a couple of theories out there.Perhaps China's nationwide poultry vaccination program launched last summer has dampened down the virus, or it could be a side effect from seeing back-to-back severe seasonal flu (H3N2 & Influenza B) outbreaks in the past 6 months (see PLoS Comp. Bio.: Spring & Early Summer Most Likely Time For A Pandemic).
Or perhaps something else is going on. Or, more likely, a combination of factors.Similarly, after last year's record setting H5N8 epizootic in Europe - where more than 2,000 outbreaks were reported - Europe has seen very little bird flu activity this winter, and what they have seen has come from a reassorted H5N6 virus - not H5N8 (see UK: DEFRA Updates Avian Flu Risk Assessment - Jan 22nd).
While these unexpected lulls in avian flu reports are welcome - we've seen them before - and they've all been followed by a resurgence in novel flu outbreaks.As the chart at the top of this page illustrates, H7N9 had been on the decline in China after it's second season (2013-14) highs. Then, in December of 2016, after the virus evolved into a new lineage and mutated into both LPAI and HPAI strains, it took off again.
H5N1 had been on a 5-year decline since 2010, when over the winter of 2014-15 it erupted in Egypt to produce the biggest human outbreak on record (see chart below). Since then, we've only seen scattered cases, but the virus is still out there, and still evolving.
While the above examples are all for avian flu, we've seen similar patterns in swine flu viruses as well. After 7 years of only seeing sporadic cases, in 2012 we saw a record number (n=309) of swine variant (H3N2v, H1N2v & H1N1v) cases reported across the United States, which was followed by four years of either single-digit or low double-digit numbers.
Last year, the CDC reported:
A total of 67 variant virus infections have been reported to CDC during 2017. Sixty-two of these have been A(H3N2)v viruses (Delaware , Iowa , Maryland , Michigan , Nebraska , North Dakota , Ohio , Pennsylvania , and Texas ), one was an influenza A(H1N1) variant [A(H1N1)v] (Iowa ) virus, and four were influenza A(H1N2) variant [A(H1N2)v] viruses (Colorado  and Ohio ). Six of these 67 infections resulted in hospitalization; all patients have recovered.
Whether this is a trend, or simply a blip in the numbers, is something only time will tell. But it illustrates how variable these yearly totals tend to be.
Over the weekend the WHO released their latest Influenza at the human-animal interface report which covers the period from December 8th-January 25th - during which time only 4 human infections were reported to the WHO.
Influenza at the human-animal interface
Summary and assessment, 8 December 2017 to 25 January 2018
- New infections1: Since the previous update, new human infections with avian influenza A(H5N6) and A(H7N9) viruses, and influenza A(H1N1)v and A(H3N2)v2 viruses were reported.
- Risk assessment: The overall public health risk from currently known influenza viruses at the human-animal interface has not changed, and the likelihood of sustained human-to-human transmission of these viruses remains low. Further human infections with viruses of animal origin are expected.
- IHR compliance: All human infections caused by a new influenza subtype are required to be reported under the International Health Regulations (IHR, 2005).3 This includes any influenza A virus that has demonstrated the capacity to infect a human and its heamagglutinin gene (or protein) is not a mutated form of those, i.e. A(H1) or A(H3), circulating widely in the human population. Information from these notifications is critical to inform risk assessments for influenza at the human-animal interface.
Avian Influenza Viruses
Avian influenza A(H5) viruses
Since the last update on 7 December 2017, one new laboratory-confirmed human case of influenza A(H5N6) virus infection was reported to WHO.
A 3-year-old female resident of Fujian Province, China, developed symptoms on 19 December 2017. She was diagnosed and treated as an outpatient and has recovered. The patient had exposure to live poultry before illness onset; no further human cases were reported among her close contacts. Additional information on the virus from the case is anticipated.
A total of 19 laboratory-confirmed cases of human infection with influenza A(H5N6) virus, including six deaths, have been reported to WHO from China since 2014.4
Avian influenza A(H7N9) viruses
Since the last update on 7 December 2017, one new laboratory-confirmed human case of influenza A(H7N9) virus infection was reported to WHO. A 72-year-old male resident of Xinjiang Uyghur Autonomous Region, China, developed symptoms on 3 January. He was hospitalized on 6 January and passed away on 10 January 2018. The patient had exposure to a live poultry market before illness onset; no further human cases were reported among his close contacts. Additional information on the virus from the case is anticipated.
Since 2013, a total of 1566 laboratory-confirmed cases of human infection with avian influenza A(H7N9) viruses, including at least 613 deaths7, have been reported to WHO (Figure 1). If the incidence of human cases follows the trends seen in previous years, the number of reported human cases may rise over the coming months. Further sporadic cases of human infection with avian influenza A(H7N9) virus are therefore expected in affected and possibly neighbouring areas.
The agricultural authorities in China have announced that vaccination of domestic poultry against infection with avian influenza A(H7) viruses has commenced, in addition to the ongoing poultry vaccination program against avian influenza A(H5) viruses.8 Overall, the risk assessment has not changed.
Swine Influenza Viruses
Influenza A(H1N1)v viruses
On 16 January 2018, Switzerland informed WHO of a case of human infection with a swine influenza A(H1N1)v virus. The case is a male farm worker, aged 48, living in Switzerland. On 20 December 2017, he presented with mild acute respiratory symptoms 8 days before a nasal swab was collected. The virus isolated from this human case was partially sequenced and was closely related to the European avian-like swine influenza A(H1N1) viruses circulating in swine in Europe. Samples from the swine at the farm where the case worked also tested positive for influenza A viruses, and are currently under characterization. No additional human cases related to this event were reported. Human cases infected with swine influenza viruses have been detected in Switzerland in 2003, 2009, 2010, 2011 and 2016. Swine influenza A(H1N1) viruses are endemic in pig populations and circulate among swine in many regions of the world.
Influenza A(H3N2)v viruses
Since the last update on 7 December 2017, one human infection with an influenza A(H3N2)v virus was reported in the U.S. in the state of Iowa.9 The case reported contact with swine in the week prior to illness onset. The case was not hospitalized and has fully recovered, and no human-to-human transmission was identified. Since human infections with novel influenza A viruses became nationally notifiable in 2005, 434 human infections with influenza A(H3N2)v viruses have been reported to the U.S. CDC and 62 of these occurred in 2017.6
Most cases are associated with mild illness, although several cases have been hospitalized and one case was reported as a fatal case (in 2012). Swine influenza A(H3N2) viruses are endemic in pig populations and circulate among swine in many regions of the world.
While there remain large swaths of the globe where avian flu surveillance and reporting remains `sub-optimal' - and only about 1/3rd of the countries of the world currently self-report they have met the core requirements of the 2005 IHR (see Adding Accountability To The IHR) - this winter's drop in reported human infections appears legitimate.
As to the reasons behind it, and how long it will last, we'll simply have to wait and see.