COVID-19(214) SARS-CoV-2(201) variant(63) Vaccine(50) coronavirus(49) pandemic(49) Travel(44) outbreak(38) quarantine(36) delta variant(34) Delta(34) country(30) omicron(27) Transmission(26) variants(26) travel.(26) vaccination(26) neutralization(24) India(22) China(21) COVID-19 vaccine(21) lockdown(21) acute respiratory syndrome(20) variants of concern(20) sera(19) Epidemiology(19) Infection(18) Abstract(18) Lineage(18) Result(18)