The humoral immune response in children receiving aerosol measles vaccine has been demonstrated with seroconversion rates ranging from 33% to 100% [10
]. School-age children who received aerosol measles vaccine as a second dose showed an excellent response in seroconversion and seropositivity rates [16
]; when the same dose was given as a primary immunization to 12-month-old children the response to aerosol vaccine was similar to responses seen in children immunized via the subcutaneous route [17
]. However, when this same vaccine strain and dose was subsequently given to 9-month-old children, the immune response was lower than that seen in 9-month-olds given subcutaneous vaccine [18
There is limited information on vaccinating 9-month-old children using aerosol EZ measles vaccine and scant data defining the minimum immunizing dose needed for this age group when the aerosol route is used; 2 successful studies in 9-month-old children used aerosol EZ measles vaccine with potencies of 104.7
]. Based on these studies, we hypothesized that technical changes to increase the dose administered could potentially improve immune responses.
Using the manufacturer's specifications (10 doses in 5 mL), we increased exposure time from 30 seconds to 2.5 minutes, providing an estimated retained dose of 1416 viral particles. This dose is higher than that provided by a single 30-second burst used previously (vaccine potency 104.28
, estimated retained dose of ~646 viral particles) [18
]. Since the current minimum dose for subcutaneous measles vaccination is 1000 TCID50
], our calculated estimate provided a good rationale for testing this approach and supported our hypothesis that a longer aerosol exposure would improve the retained dose, enhance immunogenicity, and result in a response equivalent to that seen after subcutaneous immunization.
Using the surrogate marker for protection for measles (measles neutralizing antibodies ≥120 mIU/mL, detected by PRN assay) [26
], we found that 95% of children in the aerosol group achieved this level of measles antibody, which was not significantly different from the 91% observed in the subcutaneous vaccine group. GMCs increased in both groups and were above the protective concentration 3 months after primary vaccination. Moreover, when MMR was administered at 12 months of age, we observed a 4-fold increase in GMCs in both groups, suggesting that aerosol measles vaccine given before 12 months of age primed the immune response for ananamnestic response when a second dose was administered at 12 months [29
Previous studies using [3
H] thymidine incorporation to detect measles specific T-cell responses gave no information about types of T cells proliferating in response to measles antigens. The present study also describes similar percentages of vaccines with measles specific lymphoproliferative T-cell responses following either measles vaccine subcutaneously or a 2.5-minute exposure to aerosol measles vaccine. We were able to extend these findings by looking for the induction of measles-specific memory cells by incubating PBMCs in the presence of measles antigen for 5 days, on the rationale that memory cell induction takes place several days after initial antigen contact (especially in vitro), and 5 days of incubation would allow clonal expansion of these memory T cells. Afterward, cells were stained with antibodies directed to surface proteins that are known markers of peripheral memory effector T cells [22
]. Using this new method, we were able to detect an increase in the proportion of CD8+ memory T cells (CD62L− CCR7−) and in CD4+ memory T cells (CD62L– CCR7–) 3 months after vaccination and demonstrated persistence of these subsets 6 months after the initial dose of measles and 3 months after a dose of MMR. An increase in memory B cells (CD19+ CD27+) in both groups was also observed 3 months after vaccination. Unexpectedly, mean cell frequencies of memory B cells decreased 6 months after primary measles vaccination (ie, 3 months after a dose of MMR vaccine) in both vaccine groups. This phenomenon could be due to a homing of specific memory B cells to lymphoid organs and lower number in peripheral blood or to a higher proportion of memory B cells rapidly differentiating into plasma cells upon contact with measles antigen, but the observation was made after acquiring all the cells, and we unfortunately did not stain for plasma cell surface markers (CD138 SYNDECAN I). This is the first report of the induction of measles-specific peripheral memory CD8+ T, CD4+ T, and B cells in response to the aerosol measles vaccination.
Previous sero-epidemiologic studies suggest that some individuals are protected against measles, even when measles antibody titers are low or undetectable by conventional assays [30
], and some vaccine recipients have a higher or longer lasting cellular response in the absence of a humoral response [31
]. Whether these individuals have long-term protection is not known. However, given the importance of cell-mediated immunity in controlling viral infections, children with this pattern of response may be protected against measles infection. CMI responses probably serve as a marker for individual protection, but whether they play any role in preventing infection, transmission, or enhancing herd immunity is not understood and requires further study.
Thus the assays for T-cell immunity with [3H] thymidine and neutralizing antibodies can segregate infants into 4 immune response patterns using a threshold of measles neutralizing antibody of ≥120 mIU/mL along with detectable measles T cell proliferative responses (SI ≥3.0), as illustrated in . In contrast to the past study in 9-month-old children, where 30 seconds of exposure to aerosolized measles vaccine resulted in 50% of the children failing to develop detectable measles PRN antibodies and with SI <3, a longer exposure time elicited a pattern of immune response that was similar to that seen after subcutaneous administration (). In the present study, the majority of infants given aerosol measles vaccine developed both T-cell and B-cell responses or humoral response only, while a very low proportion had cellular responses only and a mere 3% in the aerosol group and 2% in the subcutaneous group had no detectable response.
Surprisingly, only 4% of 9-month-old infants in our present study had maternal passive antibodies to measles, even though Mexico experienced a measles epidemic in 1989–1990 [5
]. This finding suggests that more children are born to vaccinated younger mothers instead of mothers with immunity from natural measles infection [6
Although there was a 5-fold increase in exposure time for aerosolized measles vaccine compared with previous studies, no serious adverse reactions were observed, and the frequency of symptoms occurring after vaccination were equivalent between aerosol and subcutaneous groups.
In summary, this study demonstrated that increasing exposure time is an approach that may improve immunogenicity in 9-month-old infants given their primary measles immunization by the aerosol route; furthermore, this time of exposure was well tolerated. We also detected measles-specific memory CD4+ T, CD8+ T, and B cells in response to the aerosol measles vaccine in 9-month-old children, which has not been previously reported. This method of vaccine administration is potentially adaptable for both mass campaigns and individual dosing with practical advantages for measles vaccination campaigns throughout the world.12