Duration of Immunity: An Anamnestic Response 14 Years After Rabies Vaccination With Purified Chick Embryo Cell Rabies Vaccine

Abstract

ecent cases of human rabies imported to Eu-rope by travelers visiting India and Morocco demonstrate the relevance and value of receiving proper education regarding rabies prevention, in-cluding both preexposure vaccination and postex-posure prophylaxis, prior to leaving for canine rabies – endemic countries. 1,2 This is especially rele-vant for travelers planning on visiting countries in Asia and Africa where the majority of human rabies cases occur. 3 Modern cell culture vaccines recommended by the World Health Organization (WHO), including purifi ed chick embryo cell vac-cine (PCECV), purifi ed Vero cell rabies vaccine, and human diploid cell rabies vaccine (HDCV), provide a safe means by which travelers can be pro-tected in the event of exposure to rabies virus. These vaccines have been licensed for several decades, but there are limited published data regarding the length of time after a person has received his or her primary rabies vaccination, for which they will continue to develop an anamnestic response after a booster dose of rabies vaccine has been adminis-tered. 4,5 The vaccination recommendations for a person who has been vaccinated previously with a cell culture rabies vaccine and is subsequently exposed to a rabid animal include the administra-tion of two booster doses of vaccine (one dose on day 0 and the other on day 3) without the adminis-tration of rabies immunoglobulin (RIG). However, the limited amount of available data has led to some confusion regarding booster recommendations in previously vaccinated persons who are subse-quently exposed to rabid animals. 6,7 To determine how long individuals would be able to mount an anamnestic response after primary vac-cination, we contacted all veterinarians who had ini-tially received PCECV as students enrolled in a clinical trial initiated in 1986. 8 The veterinarians who agreed to participate in this booster study veri-fi ed and confi rmed that they had not received a rabies booster vaccination for 14 years since the clin-ical trial was completed, and all agreed to provide their serum samples for serological evaluation before and after receiving a booster dose of rabies vaccine. In the initial clinical trial conducted in 1986, 78 individuals were vaccinated with either HDCV or PCECV intramuscularly or intradermally as a pri-mary three-dose series. 8 In 1988, 68 of the 78 subjects received a single 1.0 mL of PCECV in tramuscularly. After the booster dose in 1988, the mean titer for subjects who received intradermal dosage initially was 1,106 (range: 270 – 6,800) and for subjects who received intramuscular dosage for their initial series was 1,859 (range: 1,100 – 9,500). For comparison purposes, a reciprocal titer of 1:25 is approximately equal to a titer of 0.5 IU/mL. Fourteen years later, 15 of those who initially received PCECV and a dose of intramuscular booster 1 year after their primary se-ries and who had not received any additional rabies vaccination since the 1988 booster were enrolled in this study. After approval of the study by the ethics committee and obtaining informed consent from the subjects, a blood sample was obtained and a single