Pandemic flu: will there be a second wave?

Rates of swine flu are levelling off in the northern hemisphere as summer progresses. The question now is whether we’ll have a second wave this winter, and if so how bad it will be. Two evolutionary virologists writing in JAMA are cautiously reassuring (JAMA 2009;302:679-80). Looking back over the 14 or so pandemics since 1510, they say that pandemic flu has never been able to infect the entire population at once, and although it tends to recur after a first wave, it eventually adopts the familiar seasonal flu pattern.

As for A/H1N1, they say its modest transmission efficiency and its arrival in the northern hemisphere’s early summer, as well as the degree of pre-existing population immunity from previous flu viruses and vaccines, all give reason to hope for a more indolent pandemic course and fewer deaths than in past pandemics. They don’t think it’s inevitable that the virus will mutate into a more virulent or contagious strain.

A second wave of A/H1N1 during the northern hemisphere’s winter would coincide with its regular bout of seasonal flu, something the southern hemisphere is experiencing at the moment. Melissa Sweet asks what lessons we can learn from Australia (doi:10.1136/bmj.b3317), and finds contradictory views among experts there. Some think the final death toll of A/H1N1 will be lower than for seasonal flu, noting that the big difference between now and 1918 is the availability of effective antibiotics. But others warn against being too dismissive. They are seeing respiratory failure of an unprecedented severity in young people, sparking concerns about how well intensive care will cope.

For me the wisest voice is that of Robert Booy. “We had both more severe disease and a huge amount of very mild disease, and people either played it one way or the other, and in doing so they either underplayed it or overplayed it, and the truth was somewhere in the middle.” This fits with Heath Kelly’s advice among a list of top tips for the northern hemisphere: “Be aware of the pandemic paradox: a high proportion of asymptomatic and mild cases but serious disease in the overweight and pregnant women.”

If swine flu does come back with a vengeance, the UK may already have shot one of its bolts. Despite WHO advice to the contrary, in England the government opted for relatively indiscriminate use of antivirals after self diagnosis. Could the government really not have provided more balanced information on the likely benefits and harms of antivirals and been more parsimonious? Matthew Shun-Shin and colleagues updated and expanded an earlier Cochrane review on the effectiveness, safety, and tolerability of oseltamivir and zanamivir in seasonal influenza in children (doi:10.1136/bmj.b3172), information that must have been available to ministers when they were making their plans. The evidence suggests that these two agents are of limited efficacy in children and are associated with a significant risk of side effects, mainly vomiting. Coupled with the risk of resistance and the financial cost of widespread use, these findings have led to calls to cut back the use of antivirals in mild flu and flu-like illness.

These calls are hard to ignore. Indeed, Hugh Pennington, emeritus professor of bacteriology at the University of Aberdeen, has told the media that Tamiflu may be useless by the autumn because of current indiscriminate use (http://tinyurl.com/qnsm4s). According to news reports, ministers thought it would be publicly unacceptable to withhold antiviral drugs, even after it became clear that swine flu was generally a mild illness. Yet we know it’s possible to educate patients and the public about the need to avoid unnecessary use of drugs, as last week’s BMJ showed (BMJ 2009;339:b2885).