However, it was previously believed and suggested that the vaccination would only provide effective coverage of between five and seven years, but subsequently it has been appreciated that long-term immunity derives from immunological memory which outlasts the loss of antibody levels and hence subsequent testing and administration of booster doses is not required in successfully vaccinated immunocompetent individuals.
Namun, sebelumnya diyakini dan disarankan bahwa vaksinasi hanya akan memberikan perlindungan efektif antara lima hingga tujuh tahun, tetapi kemudian dipahami bahwa kekebalan jangka panjang berasal dari ingatan imunologi yang hidupnya melebihi dari kehilangan tingkat antibodi sehingga pengujian dan pemberian dosis pendorong selanjutnya tidak diperlukan pada individu imunokompeten yang berhasil divaksinasi.