The ‘Hong Kong’ pandemic killed between one million and four million people

“I have a little bird and its name is Enza. I opened the window and in flew Enza.”

– A skipping song from 1918.

There has been much reference to the 1918 Spanish Flu pandemic in the months since Covid-19 took hold across the world. More than 80,000 Irish people were infected, and more than 23,000 died in several waves of infection from 1918 through 1919. The Spanish Flu was caused by an avian influenza virus; more recent flu outbreaks have included a virus that emerged from swine.

The last serious pandemic infection occurred in 1968. Although far less referenced than the Spanish version, it resulted in an estimated one million to four million deaths, far fewer than the 1918 pandemic.

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The 1968 “Hong Kong” pandemic was caused by influenza A (H3N2). When a particularly virulent influenza hits, it is like nothing else we have grown accustomed to. The normal aches and pains become prostration – people are debilitated to the point that they cannot raise their heads from a pillow.

A retired Irish GP described the rural village in which she lived and worked as being like a ghost town during the 1968 outbreak. “There was no one on the streets. Entire households were affected, with no one stirring out for weeks. Essential supplies were delivered by neighbours lucky enough not to be affected.”

The rate of house-calls rocketed; she worked 18-hour days for weeks on end before succumbing to infection herself.

Different microbes

While acknowledging the fact that influenza and coronaviruses are completely different microbes, are there any pointers to our future with Covid-19 from the 1968 flu pandemic?

Influenza H3N2, like Covid-19, is highly contagious. Differences in immunity were evident as the flu virus mutated during its global spread, as shown by the different patterns of infection and death. How much antigenic drift are we seeing already with Sars- CoV- 2?

It’s tricky and certainly not predictable, but in general the impact and outcome of a pandemic will depend on a wide number of factors which include how pathogenic the virus is, the attack rate, the severity of the clinical illness, the case fatality rate and the effectiveness of preventative measures.

In a "worst case" influenza scenario with a 50 per cent attack rate and a fatality rate of 2.5 per cent, modelling suggests we would have over 2 millions cases of disease in Ireland, giving rise to some 78,000 hospitalisations and almost 53,000 deaths.

As the 1968 epidemic progressed, initially throughout Asia, important differences in the pattern of illness and death became apparent.

In Japan epidemics were small and scattered until the end of 1968. Most striking was the high illness and death rates in the US following introduction of the virus on the West Coast. This contrasted with the experience in western Europe, in which increased illness occurred in the absence of increased death rates in 1968-1969, and increased death rates were not seen until the following year of the pandemic.

Smouldering

The 1968 pandemic has been characterised as "smouldering" – possibly due to the presence of an N2 component that had not changed from previous infections. In other words, people who had been exposed to the original retained some defence against Hong Kong flu.

We are also seeing a wide variety of illness with Covid-19 across the globe. Extrapolating from the 1968 experience could it be that those who have tested positive for infection but remain asymptomatic have benefited from a previous coronavirus infection?

Or that such a prior exposure leads to a milder illness?

Yet the relative sparing of children and younger people goes against prior exposure to a coronavirus offering an element of protection.

A usable Covid-19 vaccine is some time away. Which leaves us with a range of social control measures designed to snuff out any outbreaks – something that wasn’t widely implemented during the 1968 Hong Kong pandemic.

mhouston@irishtimes.com