| CHICAGO, March 30
CHICAGO, March 30 Prior infection with West Nile
or dengue - two viruses closely related to Zika - can make Zika
symptoms worse, U.S. researchers said on Thursday.
The findings in mice, published in the journal Science,
confirm studies in cells suggesting that prior infection with
dengue could worsen the effects of Zika.
That could explain higher rates of severe Zika side effects,
such as the birth defect microcephaly, in areas such as Brazil,
where dengue is common. It also raises concerns about current or
experimental dengue vaccines by Sanofi, Takeda
Pharmaceutical Co, GlaxoSmithKline and others
because they could inadvertently make Zika infections worse.
In the study, a team of researchers at the Icahn School of
Medicine at Mount Sinai Hospital in New York injected
Zika-infected laboratory mice with very low amounts of
antibodies against dengue or West Nile.
They found that mice injected with antibodies from either
virus were more likely to die from Zika or have more severe
symptoms than mice exposed to Zika alone.
Symptoms were worst among Zika-infected mice that were given
dengue antibodies, with only 21 percent of the mice surviving.
That compared with survival rates of 93 percent among mice
infected with Zika alone.
Zika-infected mice that got the dengue antibodies also
developed severe neurological symptoms, including paralysis of
several limbs and, in some cases, total body paralysis, symptoms
that have also been seen in rare cases of adults infected with
The findings illustrate a process called antibody-dependent
enhancement already seen in people infected with one of the four
strains of dengue. This occurs when proteins made in response to
the first infection make it easier for a related virus to enter
Given the high rates of dengue antibodies in regions most
affected by Zika, the findings "suggest that pre-existing
immunity to dengue may have contributed to the rapid spread of
Zika in the Americas, possibly associated with increased viremia
and clinical symptoms, including microcephaly," the researchers
The findings may also have implications for people who
develop Zika in the United States, where more than 3 million
people have been infected with West Nile virus, said Mount Sinai
microbiologist Jean Lim, one of the study's authors.
As for the potential increased risk due to vaccines, Lim
suggested that companies may consider combining a dengue and
Zika vaccine to protect against both illnesses at once.
(Reporting by Julie Steenhuysen; Editing by Dan Grebler)