We propose that breakthrough infection after COVID-19 vaccination is defined by evidence of lower respiratory tract infection (LRI) and a positive PCR test for SARS-COV-2. Vaccinated individuals who are asymptomatic or have symptoms limited to an upper respiratory tract infection (URI) are not experiencing a breakthrough infection, although it is important to understand that these individuals may still be able to transmit virus. We also suggest that those individuals who do not mount an effective immune response to vaccination do not represent cases of breakthrough infection. *Transmission risk is likely proportionate to the viral copy number.