Fig. 1.

Research trends of various vaccine types. Values are based on PubMed results (which could be an indicator for popularity) using EndNote between 1931 and 2020, where the projected values, as indicated by an asterisk (‘*’) for 2011–2020, were calculated by multiplying the current values by the ratio 120/87.5. The total number of months is 120, and the number of months to date within the time period of interest (Jan 2011–April 2018) is 87.5. The following keywords were entered into the “title” form in EndNote when searching for PubMed research articles: subunit vaccine, live attenuated vaccine, toxoid vaccine, inactivated vaccine, polysaccharide vaccine, recombinant vaccine, and DNA vaccine. The total number of results would be different, however, if the abstract or other keywords had been queried.