Wednesday July 29 2009
A low level of pregnant women are opting for the flu vaccine
Pregnant women who develop swine flu should be given antiviral treatment as soon as possible, say the authors of a new study. The research gathered data from 13 US states between mid-April and mid-May 2009, and has shown that the rate of hospital admissions of pregnant women with flu was four times higher than among the general population.
However, there is some suggestion that doctors may be more likely to admit women if they are pregnant, and that the pattern of cases seen in the early part of an epidemic may be different to the pattern of cases seen now. This study supports the current advice that pregnant women are a high-risk group that should be offered antiviral treatment early and given priority for vaccination when a jab becomes available.
- Eleven (32%) of the 34 pregnant women with swine flu were admitted to hospital. This is four times the admission rate of the general population at the time.
- Amongst all the pregnant women, 24% were given antiviral medication within 48 hours of symptoms. However, when the researchers looked at those who died they found that they had been treated with antivirals between 8 and 15 days after symptoms, suggesting that there was some delay in starting treatment.
- The fact that doctors may be more likely to admit pregnant women with swine flu is one explanation for the higher rate of hospital admissions. However, the increased number of deaths among pregnant women also suggests that the disease is more serious in pregnancy.
- According to the researchers, vaccinating pregnant women should be a priority. They say that there is currently a low level of uptake of the seasonal flu vaccinations among pregnant women, with one 2004 study finding that uptake was as low as 14%.
- Most states changed how they reported data to the CDC from May 18, therefore the researchers were unable to calculate illness and admission rates after this date.
Where was the article published?
This research was carried out by Denise Jamieson and colleagues at the Novel Influenza A (H1N1) Pregnancy Working Group, based in the Centers for Disease Control and Prevention (CDC) in the US. The study was published in The Lancet and funded by the CDC.
What kind of study was this?
This cross-sectional descriptive study investigated the severity of swine flu in pregnant women during the first months of the pandemic. The rates of flu and complications among pregnant women (such as hospital admissions) are from the first five weeks of the pandemic (April 15 to May 18). Deaths associated with the virus are from the first nine weeks (April 15 to June 16).
The researchers also describe other features, such as the number of women who took antiviral medication, which medication they took, the length of admissions, and any delays until the flu was diagnosed. They compare these rates of complications and death with similar measures of severity in people who developed swine flu and were not pregnant.
The researchers were based at the CDC and therefore had all the data on confirmed cases up to the date when reporting changed (May 18) and state laboratories began testing samples themselves.
Confirmed cases at that time were defined as women with an acute respiratory illness and laboratory-confirmed pandemic H1N1 virus infection. The infection was confirmed by specific laboratory tests. For this study, they also included probable cases, people with an acute febrile respiratory illness who were positive for influenza A, but negative for H1 and H3 (a less specific test).
The calculations of population numbers (for the denominator part of the rates quoted) were estimated using data from the 2007 census. This put the US population at over 301 million people, with 62 million women of reproductive age (15—44 years). Making assumptions about the fertility rate and abortion rate, the researchers estimated there were 3 million pregnant women in the US at the time of the study.
What does the research say?
During the reporting period, the CDC received reports of 34 confirmed or probable cases of pandemic H1N1 in pregnant women across 13 states. Of these women, 11 (32%) were admitted to hospital.
During the first month of the outbreak the estimated rate of admission of pregnant women infected with pandemic H1N1 influenza virus was higher than it was in the general population, at 0.32 per 100,000 pregnant women (95% CI 0.13 to 0.52) compared to 0.076 per 100,000 in the general population (95% CI 0.07 to 0.09). This represented a four-fold difference.
From April 15 to June 15, six deaths in pregnant women were reported to the CDC. All were in women who had developed pneumonia and subsequent acute respiratory distress syndrome, which required mechanical ventilation.
What is the implication and importance of this?
The researchers say that “pregnant women might be at increased risk for complications from pandemic H1N1 virus infection”. They say their data lends support to the present recommendations to promptly treat pregnant women with anti-influenza drugs when they acquire infection with the H1N1 influenza virus.
There are several points to note about the study:
- In relation to the emerging pattern of disease in the UK, this data is already quite old. It is possible that the early US pattern of disease (when there were around 5500 confirmed and probable cases) may differ from the current pattern in the UK, when there are reported to be more than 50,000 new cases a week.
- Picking up women with swine flu depended on the testing regimen in place at the time, and the researchers say that, due to limitations, these methods varied by state. If women were either more or less likely to be tested than the general population, it could affect the accuracy of the study’s estimates.
- The researchers also say that healthcare workers may be more likely to admit pregnant women with flu symptoms to hospital, and that this could lead to an overestimation of complication rates.
Overall, this study provides useful support for the early treatment of pregnant women. Although the analytical methods in this study did not assess the idea that pregnant women should be given priority for vaccination, such a move is supported by the researchers. They highlight the point that there is currently a low level of uptake of the seasonal flu vaccinations amongst pregnant women, with one study in 2004 finding that it was as low as 14%.