Jeremy Laurance: Swine flu: the pandemic that ended with a whimper

Medical Life

So that's it. The swine flu pandemic of 2009 is over. The World Health Organisation (WHO) has acknowledged that the pandemic "may have peaked in parts of the northern hemisphere". In the UK, cases have been declining for four weeks and are now only just above baseline levels. Swine flu is not going to return this side of Christmas.

Can we relax? Not yet. Chief medical officer Sir Liam Donaldson says there could be another peak in January or February and, more worrying, the virus could mutate.

But suppose it does not? Will the WHO have to redefine the term "pandemic virus"? The only pandemic we have seen so far is the one at the end of the virologists' microscopes. OK, so it is a completely new flu virus of a kind never seen before – hence its title "Novel H1N1 influenza". But there has been no "pandemic", in any clinically relevant sense of the term, anywhere in the world since swine flu first broke out in Mexico last April.

Virologically speaking, swine flu may meet the criteria for a pandemic. It has targeted younger people rather than older and we have never had flu in summer before. But flu is different every year.

In June, the WHO announced a Level 6 global alert – the highest – for the first time in 40 years. It was intended to help the world prepare for an imminent lethal viral attack not seen in decades and required the mobilisation of immense resources. It hasn't happened. What began with a thunderclap is ending – for this year at least – with a whimper.

True, if the virus does mutate it could cause mayhem on a scale not yet seen. But the risk is reducing as the vaccine is rolled out – almost two million have already had the jab in the UK and millions more will get it in the next couple of months. The existing vaccine should provide some cross protection even against a mutated virus.

If we have seen the worst of the "pandemic", we can all breathe a sigh of relief. But it will leave the virologists with some tough questions to answer. Among them is this: how was it that a wholly "novel" virus turned out on closer inspection not to be so novel at all? We knew it was mild from the beginning but we thought that because it was novel it would spread widely and, even though mild, kill a lot of people by virtue of the sheer numbers infected.

That has not happened. Estimates of the population potentially infected have been reduced from 30 per cent to 10 per cent – the same as for seasonal flu. It has emerged that people aged over 50 have some immunity because, although the virus itself is novel, proteins on its surface are similar in some respects to those on H1N1 viruses circulating up to the 1960s.

If it turns out that we have used a sledgehammer to crack a nut, my question is this: how did the virologists get the scale of the threat so wrong?