Flu: Down, But Not Completely Out

# 4607

The CDC released on Friday what they expect to be their last FluView report for the 2009-2010 flu season. Influenza activity in the United States remains low.

While flu surveillance will continue throughout the summer, regular weekly reporting won’t resume until the fall.

A brief look at this last report, and a look around the globe at where pandemic H1N1 – and even low levels of seasonal H1 and H3 strains – are still circulating.

2009-2010 Influenza Season Week 20 ending May 22, 2010

This is the final report of the 2009-2010 season.
The first weekly influenza surveillance report of the 2010-2011 season (week 40, week ending October 9, 2010) will be published on October 15, 2010.

All data are preliminary and may change as more reports are received.

Synopsis:

During week 20 ( May 16-22, 2010), influenza activity decreased in the U.S.

Two (0.2%) specimens tested by U.S. World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System (NREVSS) collaborating laboratories and reported to CDC/Influenza Division were positive for influenza.

Both subtyped influenza A viruses were 2009 influenza A (H1N1).

The proportion of deaths attributed to pneumonia and influenza (P&I) was below the epidemic threshold.

Three influenza-associated pediatric deaths were reported and were associated with 2009 influenza A (H1N1) virus infection.

The proportion of outpatient visits for influenza-like illness (ILI) was 1.0%, which is below the national baseline of 2.3%. All 10 regions reported ILI below region-specific baseline levels.

No states reported widespread or regional influenza activity. One state reported local influenza activity. Guam, Puerto Rico, and 13 states reported sporadic influenza activity. The District of Columbia and 34 states reported no influenza activity, and the U.S. Virgin Islands and two states did not report.

The CDC’s Key Flu Indicators page has their latest International summary, showing flu trends in selected regions of the world. Some excerpts below, followed by a brief discussion.

2009 H1N1 Flu: International Situation Update

May 28, 2010, 5:15 PM ET

This report provides an update to the international flu situation using data collected through May 23, 2010, and reported by the World Health Organization (WHO) on May 28.

The most active areas of 2009 H1N1 influenza transmission are in the tropical regions of the Caribbean and Southeast Asia. In the tropical regions of South America, 2009 H1N1 and seasonal influenza viruses continue to co-circulate at low levels. Influenza B has been reported at low but increasing levels in certain South American countries.

Selected Highlights

• According to WHO, the majority of 2009 H1N1 virus isolates tested worldwide remains sensitive to oseltamivir, an antiviral medicine used to treat flu. Among 2009 H1N1 isolates tested worldwide, 290 have been found to be resistant to oseltamivir – 67 of these isolates were detected in the United States.

Approximately 1% of U.S. 2009 H1N1 viruses tested by CDC since September 1, 2009, have been resistant to oseltamivir.

Influenza B was reported as the predominating influenza virus accounting for 66.1% of all influenza detections in China (Hong Kong SAR), 84.4 % in the Republic of Korea and 85.1% in the Russian Federation.

Sporadic detections of seasonal influenza A(H1N1) virus were reported in China and the Russian Federation, and influenza A(H3N2) activity has been reported from China, Ghana, Kenya, and Thailand recently.

The World Health Organization’s most recent pandemic update (#102) describes the global situation this way:

The most active areas of pandemic influenza virus transmission currently are in parts of the Caribbean and Southeast Asia, where low level circulation is occurring. Except for localized areas of pandemic influenza activity in parts of Chile, there is little evidence of pandemic influenza activity in the temperate zone of the southern hemisphere.

Of note, Respiratory Syncitial Virus (RSV) is widely circulating in South America resulting in an increase in respiratory disease activity, complicating somewhat the interpretation of syndromic surveillance data from the area. RSV primarily affects children under the age of 5 years.

Seasonal influenza A viruses continue to be detected at low to sporadic levels in all regions. Influenza B has been reported in increasing but low numbers in South America, where it only recently appeared, while it is decreasing in Asia.

Of particular interest is the finding that seasonal influenza A viruses continue to be detected at low levels around the world. These are the H3 and the H1 varieties.

For months, there has been speculation that these strains might disappear completely, as has happened several times in the recent past when a novel virus has emerged.

A little pandemic history is in order.

Up until 1977, we only saw one influenza `A’ strain in circulation at a time. And that was – up until that time – thought to be the normal scheme of things.

Flu Timeline

As you can see in the chart above, in 1918 a pandemic of H1N1 supplanted whatever influenza `A’ (possibly H2N2) was circulating before that time, and for the next 40 years, it was believed to be the solitary `A’ strain in the wild.

In 1957, a new reassorted virus (H2N2) emerged with the Asian Flu, and even though it was a relatively mild pandemic, it very quickly replaced the H1N1 virus.

Again, just eleven years later, a new virus (H3N2) arrived in the form of the Hong Kong Flu, and the (now seasonal) H2N2 was no more.

Each time a new virus appeared, it drove out the competition. Exactly why? Well . . . we don’t really know why.

But that was the pattern.

Until 1977. That year an old foe, in the form of the H1N1 virus, re-appeared after a 20 year absence.

How and why it returned is a mystery, although many believe it was the result of an accidental release from a Russian research laboratory.It was dubbed the `Russian Flu’, and quickly spread among the under-20-somethings who had no immunity.

But this time things were different. It didn’t replace or drive out the existing (H3N2) virus.

The reason most commonly given is that older people were less affected by the returning H1N1 virus – since those born before 1957 had previous exposure – and so they remained a reservoir of the H3N2 virus.

The two strains (H3N2 and H1N1) co-circulated, and for the past 33 years having two main `A’ strains in circulation (along with some `B’ viruses) has been the norm.

Once again, we are faced with the introduction of a new virus, the novel H1N1, and once again it has a predilection for those born after about 1957.

A bit surprisingly, the existing seasonal `A’ strains (H3N2/H1N1) all but disappeared from surveillance reports last fall despite there being an ample reservoir of 60+ year olds out there that are less affected by H1N1 and vulnerable to the older strains.

Now that pandemic flu activity has declined, surveillance has begun to pick up sporadic cases of the older seasonal H1N1 and H3N2 viruses again.

The actual number detected remains small.

Whether this means that there remains a viable reservoir of these older influenza A strains, and we might see a resurgence of regular H1N1 or H3N2 next year, is too soon to answer.

This may simply be seasonal H3’s and H1’s `last gasp’, so to speak.

But as long as there are any of these seasonal viruses in circulation the potential for their return exists.

And some scientists believe that as the number of susceptible hosts for a flu virus diminish, evolutionary pressures build upon the virus to mutate if it is to survive.

Whether that theory holds true this time is, again, too soon to say.

So, as this flu season comes to an end in the Northern Hemisphere, we are left with a bit of a cliffhanger. One filled with many unanswered questions and numerous possibilities for next fall.

While we’ll be watching the tropics and the southern hemisphere for clues over the next few months, solid answers are likely to remain elusive until we actually see what happens next fall and winter.

Stay tuned.

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