In a previous paper (1) it was shown that an effective homologous immunity to the McCoy strain of Plasmodium vivax may endure for more than three years. In 1938, the availability of the same test patient, together with another patient whose primary attack was practically contemporaneous, provided an opportunity for repeating the observation after an interval of approximately seven years since the primary attack.
The previous inoculation experiences of these patients may be summarized as follows:
(a) Patient 14. This patient was originally included in our second series of inoculations. On August 12, 1931, he was inoculated by three demonstrably infected mosquitoes (lot 38), containing the McCoy strain of P. vivax. Thirteen days later parasites were first detected in his blood smears, and over a period of 42 days he experienced a primary attack lasting 19 days and a recurrence lasting 15 days, both of which terminated spontaneously.