The infection occurs when the virus is inhaled or ingested through the nose or throat. The vaccines may contain some antibodies in saliva and nasal secretions that could be used to combat previous strains of the virus. Delta presents a more difficult challenge.The Delta variant appears to multiply in the early stages of the infection, when people most likely to contract the disease. It seems to do so in quantities that are at least 1,000 times greater than those infected by other variants. This defeats immune defenses in both the throat and nose.Jennifer Gommerman (an immunologist at University of Toronto) said that there is just one more virus that could overwhelm the initial firewall.The vaccines must produce long-lasting antibodies in blood and nose to prevent severe illness and infection. Dr. Gommerman stated that this is a very high expectation for a vaccine.Although breakthrough infections are unlikely to occur, data is not available.It is not clear how common these breakthrough infections are. Most estimates rely on data that predates the rise of Delta variants in the United States. It is difficult to make an accurate assessment due to the lack of testing available in the country.Dr. Abraar Karan, a Stanford infectious diseases fellow, stated, "I think that if people were tested randomly on the streets, we would find a lot of people who test positive."Experts believe that breakthrough infections are more likely after being exposed to the Delta variant of the virus than to previous forms. However, even if more recent data is available, it is still likely that the vast majority of deaths and hospitalizations from Covid-19 are in people who have not been vaccinated.