In ovo is Latin for in the egg. In medical usage it refers to the growth of live virus in chicken egg embryos for vaccine development for human use, as well as an effective method for vaccination of poultry against various Avian influenza and coronaviruses. During the incubation period, the virus replicates in the cells that make up the chorioallantoic membrane.
Advantages of In ovo method
In human vaccine development, the main advantage is rapid propagation, and high yield, of viruses for vaccine production. This method is most commonly used for growth of influenza virus, both attenuated vaccine and inactivated vaccine forms. It is recommended by the World Health Organization in managing influenza pandemics because it is high-yield and cost effective. 
In poultry, In ovo vaccination improves hatchability and efficient protection against Avian influenza (AI), Newcastle disease (ND) and Coronaviruses (Av-CoV). Seroconversion rates of chickens vaccinated as embryos ranged from 27% to 100% with ND vaccination and 85% to 100% for AI vaccination. The birds are protected before delivery to a commercial operation such as a farm, thus preventing the spread of Avian viruses. 
In Ovo Vaccination in Poultry
In ovo vaccination is carried out by machines. These machines perform a number of actions to ensure good vaccination of the chick inside the egg. Benefits of In ovo vaccination include avoidance of bird stress, controlled hygienic conditions, and earlier immunity with less interference from maternal antibodies. 
- Ricks CA, Avakian A, Bryan T, Gildersleeve R, Haddad E, Ilich R, King S, Murray L, Phelps P, Poston R, Whitfill C, Williams C. In ovo vaccination technology. Adv Vet Med. 1999;41:495-515.
- "http://who.int/vaccine_research/diseases/influenza/Flu_vacc_manuf_tech_report.pdf" (PDF). External link in
- Yibin Cai12, Haichen Song3, Jianqiang Ye12, Hongxia Shao12, Rangarajan Padmanabhan124, Troy C Sutton12 and Daniel R Perez12*. Improved hatchability and efficient protection after in ovo vaccination with live-attenuated H7N2 and H9N2 avian influenza viruses. Virology Journal 2011, 8:31 do
- Stone H, Mitchell B, Brugh M. In ovo vaccination of chicken embryos with experimental Newcastle disease and avian influenza oil-emulsion vaccines. Avian Dis. 1997 Oct-Dec;41(4):856-63
- "In-Ovo Vaccination: Benefits for Small Hatcheries".