H1N1 COUNTERMEASURES STRATEGY AND ... - PHE Home

NBSB Pandemic Influenza Working Group
Detailed Report
Modeling and Impact of Interventions—Neil Ferguson, MRC Centre for Outbreak
Analysis and Modelling
Mr. Ferguson presented modeling results based on data predating the current novel H1N1
influenza. In modeling the spread of pandemic influenza in the United States, Mr.
Ferguson and others worked from the assumption that the disease would start outside the
United States but in this case, the United States was among the first countries affected.
The models did not account for the affect of seasonality, and Mr. Ferguson said the
spread of H1N1 has been relatively slow because of the timing.
On the basis of data from past pandemics, Mr. Ferguson estimated that a national,
unmitigated pandemic would infect about one third of the population in the first one or
two waves. He anticipated a seroconversion rate of about 50–60%. Models suggest that
by the fall of 2009, about 100 million Americans will be sick, but most will have mild
illness, even if the pandemic is severe. The pandemic is likely to be concentrated and not
expected to be synchronized with seasonal influenza. The peak attack rate would be
about 1,500 cases per 100,000 people per day. However, perceptions of peak attack rates
at the county level may be higher. Some localities may see absentee rates as much as
30% higher than the national average, and those events may be significant enough to
cause disruption.
Planning for H5N1 focused on containing the outbreak in the early stages and slowing the
international spread of disease. Neither effort is relevant to the current situation. Novel
H1N1 virus caused nonspecific symptoms and by the time it was detected containment
was not possible. Objectives for controlling the pandemic now focus on mitigation
efforts to minimize morbidity and mortality, spread of disease, disruption to society, and
economic impact. However, we have more tools to address pandemic influenza and a
better understanding of the antivirals, rapid production of vaccine, and mitigation
techniques. The emphasis of U.S. pandemic planning is on targeted mitigation and, if the
pandemic proves severe, suppression until a vaccine is available.
Combining multiple interventions yields an impact that is larger than the sum of its parts.
Each intervention blocks a portion of the disease and provides layers of protection by
targeting different groups. The key issue for the United States is the population’s
relatively low tolerance to impose disruptive measures of mitigation if the perceived
severity of the disease is low. Modeling can explore the effects of combining
interventions.
Extrapolating from data on seasonal influenza demonstrates that vaccines offer high
protection (when highly matched to the virus), but data are limited on predicting the
impact of vaccination on reducing the spread of disease. Early data from clinical trials of
new vaccines may be available in July or August, but those studies will not show how
much the vaccine protects against infection, severe disease, or breakthrough disease.
Other data needed includes the number of doses required and whether adjuvants broaden
the immune response to H1N1, protect against drift, or boost response after one dose. If
the first vaccine is not available until October, how many people would already be
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