Pharmaceutical companies are racing to produce a vaccine against the new H1N1 influenza virus. It could go a long way to soften the blow of the pandemic if given early enough to at least half the population and to children first, suggests research published yesterday in Science. But the vaccine, currently expected in some countries from mid-October, looks unlikely to arrive in time to avert the epidemic in the USA this autumn, one of the study’s authors tells EHTF News.

 

“It’s too late,” says Ira Longini, Professor of Biostatistics at the University of Washington, Seattle, USA. “It will have virtually no effect at all.”

 

Longini, together with colleagues at the University of Washington, modelled the effect of vaccination on virus transmission in the USA based on information about the transmissibility of the virus gleaned from individual outbreaks in schools and communities. They used it to estimate the attack rate—a measure of the proportion of susceptible people infected with the virus—under different scenarios that account for availability, efficacy, and distribution of the vaccine, and considering the transmissibility of the virus.

 

They found the jab was most effective when administered before the onset of the epidemic. After it starts, if health authorities immunise children first in a campaign that covers 50–70% of the population overall, the epidemic could be mitigated enough to resemble a mild seasonal epidemic— as long as vaccination begins within 30 days of the epidemic starting, and before it peaks.

 

But experience of how previous pandemic influenza strains have spread suggests the autumn wave of 'swine flu' will probably begin this month and peak in October, says Longini. The US Centers for Disease Control and Prevention have said that a vaccine will not be available until mid-October, which will be too late to have any effect at the population level, he explains.

 

Production of vaccine doses has taken longer than expected, and the vaccine still has some regulatory hurdles to clear.

 

Nevertheless, there are some benefits for people who get vaccinated. “On an individual level, you could still get quite a bit of protection,” Longini says. Starting an immunisation campaign when the jab is on hand could ‘prime’ the population against future waves of the pandemic.

 

It is possible that the autumn wave of swine flu will come later than expected, adds Longini. “Influenza is unpredictable,” he points out: the Hong Kong flu pandemic of 1968 began in July and didn’t peak until December.

 

But the available evidence for the current pandemic suggests it will hit a high point sooner. “The virus appears to spread in the same way as the Asian flu pandemic in 1957,” explains Longini. The rapid spread of the novel H1N1 virus in the southern hemisphere also points to an early peak. In Japan, influenza activity has already passed the seasonal epidemic threshold, he notes, signalling a very early beginning to the annual influenza season.

 

Timing is crucial, the authors conclude. “Vaccination would need to start as soon as possible, and no later than September to be effective in mitigating the epidemic.”