In these studies, we tested the efficacy of topical PIO administration as a means to reduce virus shedding from the skin following traditional smallpox vaccination. This topical agent was highly effective at reducing or eliminating infectious virus on the skin surface and on the bandages used to cover the vaccination site. Although we did not begin PIO application until day 7 post-vaccination, in future studies it will be important to determine if topical application could occur at an earlier time point, such as 3–4 days after inoculation. Importantly, these studies show that PIO administration did not interfere with the development of cellular or humoral immune responses nor did it impede the healing process and eschar separation following smallpox vaccination.
Despite proper instruction and care of the vaccination site, contact spread of vaccinia virus and ocular infections continue to be a problem among vaccinated military personnel [38
]. This has led to secondary and even tertiary transfer of the virus in rare instances [39
]. One unfortunate example is described in a case of inadvertent household spread from a recently vaccinated US soldier that resulted in the infection of his breastfeeding wife who then spread the virus to their infant daughter who developed a vaccinia-culture positive vesicle on her labial philtrum, as well as lesions on her cheek, and tongue [40
]. To prevent further autoinoculation or ocular infection, goggles were placed on the infant and she was held in soft restraints while awake or not in her mother’s arms. Experiences like this could have been prevented if virus shedding was reduced or eliminated. Health care workers have used semipermeable dressings to reduce transmission, but this may not be financially feasible for mass vaccination programs such as that initiated by the US military. Povidone iodine ointment can be used by itself with semipermeable dressings or with simple gauze and tape to effectively and affordably reduce inadvertent spread of vaccinia to close contacts. The use of this antiviral cream may be most important during the pustular stages of the infection when dressings sometimes become inundated by the volume of exudate and leak infectious virus through the adhesive of the protective covering.
Interestingly, semipermeable dressings reduce, but do not necessarily eliminate infectious virus transmission. Recovery of vaccinia from the surface of semipermeable dressings is variable with 0% [31
], 7% [30
] or 18.2% [28
] of samples testing positive for vaccinia on the outer surface. Direct comparisons between gauze and semipermeable dressings have given divergent results in which the more occlusive dressings were either more effective [31
] or equivalent [29
] to gauze in terms of reducing virus shedding. In our studies, we measured infectious virus on the inner surfaces of the dressings and found that PIO administration resulted in complete inactivation of vaccinia. If infectious virus is inactivated under the protective dressing, then it is even more unlikely that virus shedding could occur and we would recommend PIO administration in combination with any protective dressing to further minimize the potential for inadvertent contact spread following smallpox vaccination. Another potential use for topical antivirals may also be as a form of primary prophylaxis by administering ointment on exposed skin of medical personnel or predisposed individuals with atopic dermatitis if they inadvertently come into contact with a recently vaccinated individual. Although washing with soap and water is typically recommended, some detergents (data not shown) and commercial soaps [41
] perform poorly at inactivating vaccinia virus. Because PIO administration is generally well tolerated, application of this antimicrobial ointment may be useful for treating other orthopoxvirus infections besides vaccinia. If smallpox were to be released during a bioterrorism event, PIO could be administered directly onto the skin lesions of the patients in order to reduce the risk of fomite spread in the hospital or clinic and/or during transport to designated quarantine sites. This may also be a useful approach to treating monkeypox patients during outbreaks that continue to occur in the Democratic Republic of Congo and possibly other neighboring countries [42
]. In this setting, PIO represents an affordable method for reducing virus transmission while at the same time helping to prevent secondary bacterial infections during the pustular stages of disease.
Povidone-iodine ointment does not appear to block virus replication under the skin (or eschar), but instead only inactivates virus that is released on the skin surface and comes into direct contact with it. This assumption is based mainly on one subject who failed to apply PIO at day 10 post-vaccination and instead removed the protective bandages and washed away the residual PIO with soap and water. We swabbed the vaccination site within 3 hours after realizing this oversight and recovered infectious virus (245 PFU/mL) – even though she had been negative for infectious virus (<10 PFU/mL) after administration of PIO 3 days earlier on day 7 post-vaccination. This suggests that as long as the PIO is applied at each dressing change, then infectious virus that is released on the skin surface is rapidly inactivated. However, if there is virus that is not in direct contact with the topical cream, then it will continue to replicate until the host immune system is able to clear the infection and the eschar separates. This likely explains why inactivation of surface vaccinia virus had no significant effect on the activation of cellular and humoral immune responses ().
Remarkably, Edward Jenner and his colleagues were the first to describe virus inactivation at the inoculation site following smallpox vaccination. In his second paper on smallpox vaccination [44
], Dr. Jenner realized that some of the “takes” were too robust and caused substantial alarm due to extensive inflammation. In one case (M.H., 12 years of age), he wrote, “The pustule, beginning to show a disposition to spread, was dressed with an ointment composed of hydrarg. nit.rub. and ung. cerae. The efflorescence itself was covered with a plaster of ung.hydr. fort.”
This topical application continued daily for 10 days until, “The girl, after the tenth day, when, as has been observed, she became a little ill, showed not the least symptom of indisposition. She was afterwards exposed to the action of variolous matter, and completely resisted it.”
In other words, he tested whether or not inactivation of surface virus resulted in the loss of protective antiviral immunity by directly inoculating the patient with smallpox and looking for resistance to infection. Smallpox inoculation (i.e., variolation) was the standard of care prior to development of smallpox vaccination. Jenner performed several similar trials and noted that the virus could be inactivated shortly after vaccination, “… the virus on the arm was destroyed soon after it had produced a perceptible sickening.”
and yet the children were fully protected against cutaneous challenge with smallpox, “The appearance and progress of the infected arm was, in every respect, similar to that which we generally observe when variolous matter has been inserted into the skin of a person who has previously undergone either the cow-pox or the smallpox.”
. Further details and examples were provided in Jenner’s third paper on smallpox vaccination [45
] in addition to publishing letters that he received from other physicians who also inactivated virus at the inoculation site within a few days after vaccination using vinegar and water or vitriolic acid (Joseph H. Marshall, letter to Edward Jenner [45
]) or by applying “…mercurial ointment to the inflamed part, which was repeated daily until the inflammation went off…”
(M.J. Tieny, letter to Edward Jenner [45
]). Based on these historical findings, our studies using povidone iodine ointment to inactivate vaccinia from the infected skin at the vaccination site are actually a simple modern extension of studies performed over 200 years ago. It is surprising that these findings have been largely forgotten, but still they provide further evidence that inactivation of surface virus does not interfere with the protective immunity afforded by smallpox vaccination – even in cases wherein subjects were directly challenged with smallpox itself.