To the Editor:

Data from epidemiologic studies and challenge experiments in animal models have shown that infection with the influenza virus can enhance susceptibility to infection by bacteria such as streptococcus, Haemophilus influenzae, and Staphylococcus aureus.1 However, the relationship between influenza virus and atypical bacteria — namely, Bordetella pertussis — has not been well characterized.2

In a randomized, double-blind, placebo-controlled trial of trivalent inactivated influenza vaccine conducted in South Africa in 2011 and 2012, we found that the vaccine against influenza in pregnant women who were not infected with the human immunodeficiency virus was 50.4% efficacious in preventing polymerase chain reaction (PCR)–confirmed influenza infection from the time of enrollment through 24 weeks post partum.3 Here, we present the results of retrospective testing by means of PCR for B. pertussis infection of the pharyngeal specimens obtained from the women at the time of visits to the study clinic because of respiratory illness.

Archived DNA samples collected independently of clinical presentation were tested by means of real-time PCR for the presence of the multicopy pertussis insertion sequence (IS) IS481.4 If IS481 cycle threshold values were less than 40, total nucleic acids were extracted from the corresponding respiratory specimen with the use of a NucliSens easyMag (bioMérieux) machine. The samples were then retested for IS481 and were tested in a duplex reaction for hIS1001 and pIS1001 and in a singleplex reaction for the pertussis toxin subunit S1 (ptx S1). The primers and probes used and the interpretation of PCR results have been described previously.4

Table 1. Table 1. Rate of Detection of Bordetella pertussis among Women who Participated in a Randomized, Double-Blind, Placebo-Controlled Trial of Trivalent Inactivated Influenza Vaccine.

Overall, 2116 women were enrolled in the trial, including 1062 in the influenza vaccine group and 1054 in the placebo group. Recipients were followed by weekly active surveillance for any respiratory illness from the time of enrollment through pregnancy and through 24 weeks post partum. A total of 3583 respiratory specimens were obtained from 1361 women, and 3125 of these specimens (87.2%) were tested for B. pertussis. Specimens obtained from 11 vaccine recipients tested positive for pertussis on PCR assay, as compared with specimens obtained from 26 placebo recipients (risk ratio, 0.4; 95% confidence interval [CI], 0.2 to 0.8). Furthermore, in 10 women in the vaccine group and in 16 women in the placebo group, the PCR results were indeterminate for pertussis (Table 1). The overall risk ratio, including the episodes of pertussis with indeterminate PCR results, was 0.5 (95% CI, 0.2 to 0.7).

Thirty-three episodes of pertussis among the women occurred after delivery, and specimens obtained from the infants tested positive for pertussis on PCR at the same time as the mother on five occasions (three in the vaccine group): within 22 days after the episode of pertussis in the mother on two occasions (both in the placebo group), and between 52 and 73 days after the episode of pertussis in the mother on three occasions (all in the placebo group). There were no instances in which both a mother and her infant had an episode of pertussis and the episode in the mother was detected after the episode in the infant.

Our results suggest that influenza vaccination may have had a protective effect on the rates of B. pertussis infection among adult women; however, this is a post hoc observation. We think it is unlikely that influenza vaccination would affect the ability to diagnose pertussis, and therefore this observation warrants further investigation of the possible mechanisms in the upper respiratory tract that can lead to the synergy between the two pathogens.

Marta C. Nunes, Ph.D.

Clare L. Cutland, M.D., Ph.D.

University of the Witwatersrand, Johannesburg, South Africa

[email protected]

Shabir A. Madhi, M.D., Ph.D.

Medical Research Council, Johannesburg, South Africa

Supported by the Bill and Melinda Gates Foundation (OPP1002747), the South African Research Chairs Initiative of the Department of Science and Technology and National Research Foundation in Vaccine-Preventable Diseases, and the Respiratory and Meningeal Pathogens Research Unit of the Medical Research Council. This is the New England Journal of Medicine version of record, which includes all Journal editing and enhancements. The Author Final Manuscript, which is the author’s version after external peer review and before publication in the Journal, is available under a CC BY license at PMC5810468. Disclosure forms provided by the authors are available with the full text of this letter at NEJM.org.

The contents of this report are solely the responsibility of the authors and do not necessarily represent the official views of their institutions or organizations or of the trial sponsors.