written by Tara Haelle

One of the concerns that comes up most often with live-virus vaccines is the risk of “shedding.” Since live vaccines use an attenuated, or weakened, form of the vaccine, there is a very, very tiny risk that a person recently vaccinated with such a vaccine may “shed” some of the virus to those around them, primarily those whose immune systems may be compromised, such as cancer patients. While shedding is possible with certain live virus vaccines, such as the MMR, varicella (chickenpox) and rotavirus vaccines, it’s extremely, extremely rare. Only a handful of cases have been documented with the MMR, only 5 cases have been documented out of 55 million doses of the chickenpox vaccine, and a single case report has been documented with rotavirus.

Now a new study adds to the evidence that the rotavirus vaccine is unlikely to shed the virus to those around the vaccinated individual. Despite the evidence that shedding is incredibly rare, researchers have not become complacent about continuing to be on the lookout for its possible risks. Today’s study in Pediatrics looked at the risk of vaccine-shed rotavirus transmission in one of the most vulnerable populations for any illness: infants in the neonatal intensive care unit (NICU). And the results are good news: not a single infant contracted rotavirus from a neighboring baby who had been vaccinated.

The idea behind the study was that many preterm babies may end up spending several months in NICU after being born. That means they could easily miss the window for rotavirus vaccination, which must be started within 104 days after birth, if the hospital doesn’t administer the vaccine in the NICU. Some hospitals have been wary about doing this because of the risk of viral shedding from the vaccine in the baby’s stools: they did not want to vaccinate one child but put other vulnerable infants in the same area at risk.

Children’s Hospital of Philadelphia (CHOP), however, has been administering the rotavirus vaccine to infants in NICU. They had decided that the risk of shedding to other infants was outweighed by the benefits of protecting infants from rotavirus, especially since discharged infants would be at high risk for the nasty gastrointestinal disease. It’s not too surprising that this would be CHOP’s position: their head of infectious disease is Paul Offit, the co-inventor of the rotavirus vaccine RotaTeq, the one used in this study. However, Dr. Offit was not involved in this study in any way, and the hospital receives no financial gains from use of RotaTeq.

Since the hospital already vaccinates NICU patients against rotavirus, they decided to investigate whether the vaccine had posed any risk to either vaccinated or unvaccinated infants. Between 2008 and 2010, a total of 96 babies received the rotavirus vaccine while in the NICU. The researchers checked the children’s records to look for any of the following symptoms in the week after vaccination: a fever over 100.4 degrees F, diarrhea or increased stool frequency, vomiting, abdominal distention, bloody stools, feeding intolerance or intussusception, which is a telescoping of the bowels and a possible risk of the rotavirus vaccine.

Of the group, 25% had no symptoms after the vaccine, and 51% had no *new* symptoms after the vaccine (but did have the same symptoms they had had before the vaccine). Another 24% did experience new symptoms, but further study into each case revealed that none of these were associated with the vaccine.

Next, the researchers wanted to see if the vaccine might have shed to surrounding babies that were unvaccinated. Since the CHOP NICU is arranged in “pods” of two to seven beds, ranging from 9 to 32 feet apart, the researchers looked through the records of all unvaccinated babies who shared a pod with a child who received the vaccine. They looked for any evidence of a bowel problem, any child who received abdominal imaging (X-ray, MRI, ultrasound, etc.), and any child who received intravenous antibiotics in the 15 days after their pod-mate was vaccinated. Any of these characteristics three might indicate a gastrointestinal illness such as rotavirus, and 15 days is the length of possible shedding time identified in clinical trials.

Out of 801 unvaccinated babies who shared a pod with an infant who was vaccinated, 6.4% had bowel issues, antibiotics or imaging, but most of these children had these before their pod-mate was vaccinated. Only 10 babies, or 1.2%, experienced any new symptoms in that 15-day post-vaccination period. A closer look showed that most of these babies’ symptoms were likely related to other issues: bacterial infection in three of them, feeding problems in two of them, necrotizing enterocolitis in two of them, post-surgery issues with two, and Hirschsprung’s disease in one. The last two were tested for rotavirus: results negative.

So, none of the 96 NICU patients vaccinated against rotavirus experienced any effects from the vaccine, and none of the 801 unvaccinated babies who shared the same pods with the vaccinated ones contracted rotavirus. Since preemies are at high risk for rotavirus after being discharged from the hospital, this study reveals the safety and value of vaccinating those children in the NICU before discharge, even if they’re not leaving any time soon. It will protect them after discharge, and it doesn’t put other babies in the NICU at risk.