“Hong Kong flu” in children: what this name means

“Hong Kong flu” is not a medical diagnosis but a name that has stuck in everyday speech and in the news. It usually denotes flu caused by the influenza A virus of subtype H3N2. Parents most often search for this phrase after seeing an alarming news headline, so let us start with what it actually means.

Where the name came from

The name is historical: it became attached to a flu pandemic of the last century, when a variant of the influenza A(H3N2) virus that was new at the time spread around the world. The ability to cause pandemics is a property of type A influenza viruses in general: of the four types (A, B, C, D), only A has ever led to pandemics. The word “Hong Kong” itself remained in the language, although what is being discussed has long been ordinary seasonal flu, not an event of half a century ago.

What “subtype H3N2” even means

Influenza viruses are divided into four types. Seasonal epidemics in people are caused by types A and B. Within type A, subtypes are distinguished — by two proteins on the surface of the virus: haemagglutinin (H) and neuraminidase (N). Hence designations like A(H1N1) and A(H3N2): these are not “names of diseases” but the formula of the surface proteins.

• Two influenza A subtypes routinely circulate among people — A(H1N1) and A(H3N2). In other words, A(H3N2) is one of the two ordinary seasonal subtypes, not a rarity and not a novelty.
• Type B is not divided into subtypes: it has lineages, and they also take part in seasonal illness.
• The seasonal flu vaccine is designed for several variants at once — it contains a component against A(H1N1), a component against A(H3N2) and a component against influenza B. So “the vaccine is for the wrong subtype” is the wrong way to frame the question.

What is special about this flu

The influenza subtypes do genuinely differ from one another: different ones predominate in different seasons, and that influences which age groups fall ill more often and how hard the season turns out to be. This is population-level statistics, tracked by surveillance services.

For an individual child, though, the conclusion is more modest: an illness caused by A(H3N2) runs like flu. Clinically it does not stand apart as a separate condition with special symptoms, special treatment or a special set of warning signs. Everything that applies to flu in children applies to it.

Why the subtype is discussed every season at all: the surface proteins of the flu virus change gradually — this is called antigenic drift. Because of it, a previously experienced infection or an earlier vaccination protects less well over time, which is why the seasonal vaccine composition is reviewed. Drift is the accumulation of small changes within an already known subtype, not the appearance of a new virus.

What this name does not mean

• It does not mean a new, previously unknown virus: A(H3N2) has been circulating among people for decades.
• It does not mean the illness came from a particular country this season.
• It does not mean a child's symptoms can identify the virus subtype: that is impossible from the picture of the illness.
• It does not mean that any special medicines are needed, different from the usual approach to flu.

How it looks in a child

The same as flu: more often a fairly abrupt onset with fever, headache, aching, weakness and a poor appetite; the runny nose and cough may join later. In small children the picture is usually non-specific — listlessness, refusing food, sometimes vomiting or loose stools. A detailed discussion of the course, risk groups, complications and prevention is in the main article on flu in children.

When to see a doctor

The decision rests on the child's condition, not on the name of a virus in the news. Breathing signs, marked listlessness, refusing to drink and a worsening after an improvement all need an examination regardless of which flu subtype is being discussed this season. The full list is in the warning-signs block on this page.

This page is for general information only and is not a substitute for a consultation with a doctor. A child's diagnosis and treatment are determined by a doctor during an examination.