More patients who were vaccinated against influenza early in the season got the flu compared to those vaccinated after mid-November, according to a retrospective chart review of HIV-positive patients at the Veterans Affairs health care facility in San Diego, California.
The U.S. typically experiences two peak flu periods during the season, one in late November and the second one between January and March of the following year. It takes about two to four weeks after someone is vaccinated against that year's influenza strains, for the individual to develop the highest number of antibodies against the illness. However, for patients living with HIV, the rate of protection seems to be about half that of the general population. Additionally, the immunity this confers wanes over time, dropping much more rapidly in HIV-positive patients.
The study, which was presented at IDWeek 2014, found that 70 of the 1,176 HIV-positive patients contracted influenza; of these, 37 had been vaccinated while 33 were unvaccinated. Of those vaccinated, 30 had been inoculated against influenza early (between Sept. 1 and Nov. 15), while seven had been vaccinated late (after Nov. 16).
Among the 37 patients who had a flu shot and still became ill, 70% (26 patients) had influenza late in the season (Jan. 16 - May 31), while 30% (11) became ill early in the season (Sept. 1 - Jan. 15). Among the 33 unvaccinated patients that trend was reversed, with 61% (20) contracting influenza early in the season and 39% (13) contracting the illness late. This suggests that patients who were vaccinated early compared to late were more likely to get flu later in the season, according to the researchers.
Influenza patients were identified by a positive screening for influenza or influenza-like illness (ILL, characterized by a fever over 100° F, a cough or sore throat, with no other known cause for the symptoms) or by a prescription for oseltamivir (Tamiflu), rimantadine (Flumadine) or amantadine (Symadine, Symmetrel) -- medications usually prescribed for influenza treatment. The median age of all 70 HIV-positive patients who contracted influenza was around 48 years and all of them were male.
Of the 18 vaccinated and unvaccinated HIV-positive patients who contracted influenza early in the season, 14 (78%) had a viral load of less than 50 copies/mL, while four (22%) had a viral load of 50 copies/mL or more. That trend was somewhat less pronounced among the 52 patients who became ill late in the season, but a slight majority (27 patients, 52% of those who had flu later) had viral loads below 50 copies/mL, while 23 (44%) had viral loads greater than or equal to 50 copies/mL.
Vaccinated patients in all CD4 cell count ranges (less than 200 cells/mm3, 200-499 cells/mm3 and 500 or more cells/mm3) were more likely to become ill 100 days or more after their vaccination than earlier (100%, 71% and 56%, respectively, of vaccinated patients in each of the CD4 ranges who contracted influenza did so 100 days or more after their vaccination date).
According to the current study's authors, these results raise a few questions about administering flu shots to HIV-positive patients:
- Should such patients be vaccinated again later in the season?
- Should they receive a higher dose of vaccine in the first place?
- When is the best time to vaccinate HIV-positive patients?