Transmission of a live influenza A virus vaccine in commercial pre-weaned pigs

The Plos One journal just published a new study by Dr. Gustavo Lopez and the Torremorell lab. The objectives of the project were to  1)assess the onset and duration of live attenuated influenza vaccine shedding in vaccinated pre-weaned pigs, 2)investigate the transmission of the live attenuated influenza vaccine from vaccinated to non-vaccinated pigs under field conditions and 3)evaluate whether the live attenuated influenza vaccine could be aerosolized and detected in the environment.


Thirty-three litters were selected in three different farrowing rooms of a commercial 5,200-head farrow-to-wean farm.
Out of the eleven litters in each of the room:

  • 7 were direct contact litters in which all piglets except for 3 were vaccinated using a live attenuated influenza vaccine.
  • 4 litters served as sentinels in which none of the piglets received the vaccine.

To monitor influenza shedding and transmission, six piglets from each of the direct contact litters were selected so that three received the vaccine and three did not, serving a direct contact. Additionally, three piglets were selected in each of the sentinel litters. Therefore, a total of 162 piglets were monitored over the course if this study.

Status of the herd was checked by taking oropharyngeal swabs from the previous cohort of weaned pigs. Individual nasal swabs, udder wipes and surface wipes were collected at  0, 1, 2, 3, 4, 6, 8 and 12 days post-vaccination. Environmental samples including air samples and airborne particle deposit were collected 1, 2, 3, 4, 6 and 8 days post-vaccination. Lastly, blood samples were taken from all piglets at days 0 and 15.

Rt-PCR was performed on the following samples: Nasal swabs were pooled by three within litters, and by vaccination status. Udder skin wipes, surface wipes, air and particle deposition wipes were tested individually. Positive samples were further characterized by virus isolation and whole genome sequencing.

Piglet sera were tested by ELISA and hemagglutination inhibition.


Oropharyngeal swabs from weaned pigs in the previous cohort and nasal swabs collected at day 0 were all negative.

100% of the nasal swabs from the vaccinated pigs were influenza positive at day 1 post-vaccination. The number of influenza positive litters decreased daily and the last positive vaccine RT-PCR was detected on day 6. However, the direct contact non-vaccinated pigs tested positive only at day 1 and 4. Sentinel litters indicative of indirect contact transmission tested influenza positive at day 2 and 12 post-vaccination. All of the four isolates were sequenced and confirmed to be vaccine strains with more than 92% homology between gene sequences.

At 3 days of age, all pigs had antibodies against influenza most likely from maternal immunity.  The number of pigs antibody positive decreased at 18 days of age with values closer to the negative cutoff value. There was no difference between the vaccinated and non-vaccinated groups.

For more detail, read the article available on the journal’s website in open access.


Influenza A virus (IAV) is one of the most important respiratory viruses affecting pig health and vaccination is the most common strategy to control influenza infections. In this field study we assessed the onset and duration of shedding of a live attenuated influenza virus (LAIV) vaccine, its ability to transmit to non-vaccinated pigs and whether the LAIV could be aerosolized and detected in the environment. Thirty-three litters (n = 33) of a farm using the LAIV vaccine were selected for the study, a subset of them (n = 12) were left unvaccinated and a subset of piglets (n = 3) in vaccinated litters were also left unvaccinated to serve as sentinels. Selected piglets from the litters were sampled multiple days post vaccination (DPV) by collecting nasal swabs and blood, and were tested using a LAIV vaccine specific RT-PCR assay and hemagglutination inhibition assay against the LAIV strains respectively. Environmental specimens consisting of air and surface wipes were also collected. One hundred percent (21/21) of the vaccinated litters tested LAIV positive 1 DPV and until 6 DPV. In contrast, only five (5/33) of the thirty-three non-vaccinated pigs tested positive during the course of the study. Viable LAIV was confirmed in vaccinated pigs by cell culture and whole genome sequencing. In addition, low levels of LAIV RNA (RT-PCR Ct values ranging between 33 and 38) were detected in all air specimens collected on the day of vaccination and until 6 DPV (3/10). Pigs had maternally derived antibodies reactive against the LAIV strains which may have influenced the degree of shedding observed. Under the conditions of this study, shedding of the LAIV from vaccinated pigs was limited in time, resulted in minimal transmission to non-vaccinated pigs and was detected in low levels in aerosols collected in the vaccinated rooms likely influenced by the presence of maternally derived antibodies against the LAIV strains.