Researchers studying the impact of the flu vaccine in
pregnancy have found a possible link between miscarriage early in pregnancy in lady
who received the flu vaccine 2 years in a row.
It is the first study to identify a potential link between
miscarriage & the flu vaccine & the first to assess the effect of
repeat influenza vaccination & risk of miscarriage. The findings suggest an
association, not a causal link, & the research is too weak &
preliminary, experts said, to change the advice that pregnant lady should get a
flu vaccine to protect them from influenza, a deadly disease that can cause
serious birth defects. But the study is prone to raise questions about the
safety of the vaccine as flu season gets underway.
“I think it’s really important for lady to recognize that
this is a possible link, & it is a possible link that needs to be studied &
needs to be looked at over more [flu] seasons,” said Amanda Cohn, senior
adviser for vaccines at the Centers for Disease Control & Prevention, which
funded the study.
“We need to recognize if it’s the flu vaccine, or is this a
group of lady (who happened to receive flu vaccines) who were also more likely
to have miscarriages,” she said.
Health officials say they recognize that the information may
be of concern to pregnant lady. They advised pregnant lady to talk to their health-care
providers for the most accurate information & to determine the best timing
for a flu shot.
The CDC, the American College of Obstetricians &
Gynecologists & the study authors continue to recommend that pregnant lady
get a flu vaccine during any stage of pregnancy because of the danger influenza
poses to lady & their developing babies. Vaccination during pregnancy is
also the most effective strategy to protect newborns, experts say, because the
flu vaccine is not approved for use in infants younger than six months.
Lots of previous studies have shown that flu vaccines can be
given safely during pregnancy, including numerous studies that found no link
between flu vaccination & miscarriage.
The new findings were part of an observational study
published Wednesday in the journal Vaccine. The researchers who conducted the
study emphasize that it is not a reason to avoid the flu vaccine, even for
pregnant lady.
“Science is an incremental process, & a lot of people
don’t recognize that very seldom does a single study provide a definitive
answer that can lead to changes in recommendations,” said Edward Belongia
Scientists at Marshfield compared 485 pregnant lady, ages 18
to 44, who had a miscarriage to 485 pregnant lady of similar ages who had
normal deliveries during the flu seasons of 2010-2011 & 2011-2012. Of the lady
who miscarried, 17 had received flu vaccine in the 28 days before the
miscarriage, & had also been immunized the prior flu season. Most
miscarriages occurred in the first trimester, but several occurred during the
second trimester. The median age of the fetus at time of miscarriage was 7 weeks.
By comparison, of the lady who had normal deliveries, four
who had received the flu vaccine in the preceding 28 days had also been
vaccinated during the previous flu season.
“We only saw the link between vaccination & miscarriage
if they had been vaccinated in the season before,” said James Donahue
Marshfield researchers conducted a similar study among
pregnant lady during the 2005-2006 & 2006-2007 flu seasons, & found no
association between the flu vaccine & miscarriage.
The Marshfield study had several limitations, including the
small number of lady who had miscarriages & who received vaccinations 2
years in a row.
The authors also said the results could be biased if lady
who sought care for their miscarriages were also more prone to have received
flu vaccinations. Miscarriages, which are among the most challenging birth
outcomes to study, often occur early in pregnancy & don't necessarily come
to the attention of health-care providers — or the lady themselves if they
miscarry before they realize they are pregnant.
If lady who routinely get flu vaccines are also more prone to be aware
of pregnancies earlier than other lady or more prone to seek care before or
after a miscarriage, that could explain the study findings.
If the flu vaccine did somehow make miscarriage more likely
during the years in the study, a possible explanation could lie in the makeup
of flu vaccine. As a result of the 2009 H1N1 swine flu pandemic that killed
hundreds of thousands of people around the world, including more than 12,000 in
the United States, vaccine manufacturers developed vaccines to protect against
the new H1N1 strain, which was different from viruses that circulated before
2009.
Flu vaccinations of pregnant lady increased substantially
during & after the pandemic. The authors speculate that the association
they observed — if it is real — may be related to an immunological response to
having been vaccinated in two consecutive influenza seasons. The composition of
the vaccines to protect against H1N1 was identical in 2010-2011 &
2011-2012.
“Scientifically, it is unclear why this would occur,” said Haywood Brown
“Additional studies
are needed to address the concern raised by this study,” he said. “In
evaluating all of the available scientific information, there is insufficient
information to support changing the current recommendation, which is to offer &
encourage routine flu vaccinations during pregnancy regardless of the trimester
of pregnancy.”
The CDC is participating in an ongoing study investigating
pregnant lady who received the flu vaccine during three more recent flu
seasons, starting in 2012-2013. Results are expected late next year or 2019.
“This is exactly why we study these things to make sure
vaccines are safe & effective,” said the CDC's Cohn .
Since 2004, the CDC & other organizations have
recommended routine flu vaccination for pregnant lady regardless of their stage
of pregnancy.
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