The new recommendations apply to both men and women, and should be followed even if neither of the partners have shown symptoms of Zika and regardless of whether they are trying to conceive or not. Common symptoms of a Zika infection include rash, fever, headache, muscle and joint pain, and on occasion red eyes that appear similar to conjunctivitis.
This is a huge change to the WHO's prior guidance in June, which only applied to men who were not showing symptoms, and suggested abstaining or practicing safe sex for a mere eight weeks after travel to an area where Zika is currently circulating.
The WHO said the change in counsel comes because of additional studies that have documented the presence of Zika virus in semen for much longer than previously known.
Until June 7, "the maximum documented time was 62 days" said the WHO, but Zika virus particles have "now been found in semen for 188 days."
Since the studies only found viral particles, not the infectious virus itself, the WHO believes its "calculation for six months is conservative" and does not intend to extend the guidance past six months at this time.
The WHO said it also took into consideration new studies that have documented the transmission of Zika between sexual partners who had no symptoms of the disease. At least 80% of people infected with Zika do not show symptoms, and may never know they have the disease and could be contagious.
Another concern, according to the WHO, centered around a case report
that showed the virus might hide in a woman's genital track. In June, researchers announced that Zika RNA had been found in a woman's vaginal fluids and cervical mucous after the virus had left her bloodstream.
The Centers for Disease Control and Prevention is still counseling couples to abstain or practice safe sex for eight weeks after returning from an area of active Zika transmission. Women with Zika-like symptoms should do so for a full eight weeks after symptoms started, but men should wait for a full six months after symptoms appear.
However, according to CDC spokesperson Tom Skinner, the agency "is currently reviewing all the available evidence and is in the process of updating its interim guidance related to pregnancy planning and the timing of pregnancy after possible exposure to Zika virus and prevention of sexual transmission of Zika virus."
The CDC defines safe sex as the use of either male or female condoms, and says it applies to oral, anal and vaginal sex acts, including the use of sex toys.
According to the WHO, safe sexual practices include "correct and consistent use of male or female condoms, non-penetrative sex, reducing the number of sexual partners, and postponing sexual debut."