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transmission

Transmission of Monkeypox Virus Through Contaminated Objects

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by Gertrud U. Rey

Image by macrovector on Freepik

Recent news headlines are fueling public fears about possible transmission of monkeypox virus through contact with contaminated objects like bedding or clothing. However, data generated using environmental sampling methods indicate that the likelihood of this type of transmission is very low.

In a study involving a two-person household in Utah in which both residents were infected with monkeypox virus, the investigators entered the home to obtain samples while the patients were present. The investigators then identified and swabbed various objects that had been touched frequently; including furniture, toilet handles, light switches, and remote controls. They subsequently processed the swab samples and performed PCR and cell culture analyses to detect the presence of monkeypox virus DNA and infectious monkeypox virus. Out of 30 samples tested, 21 yielded positive results by PCR, confirming that these 21 objects were contaminated with monkeypox virus DNA. However, none of the PCR-positive samples yielded any detectable virus in cell culture, suggesting that there was no infectious virus on any of the tested surfaces in that household.  

A second study involved a one-person houshold in Texas whose resident had been hospitalized with monkeypox virus infection. On day 15 after the patient had left the home, the investigators entered the house and obtained 31 swab samples from various household objects similar to those tested in the Utah home. PCR analysis revealed that 27 of the tested objects were contaminated with monkeypox virus DNA. Cell culture analysis showed that 7 of these 27 samples also contained viable virus, with six of the virus-positive samples originating from porous surfaces like bedding and clothing, and only one originating from a non-porous surface – the top of a coffee table. When the authors determined the concentration of virus in each of these seven samples using a standard titration assay, they found that only one of the samples produced a quantifiable amount of virus – a swab from an article of clothing that had been in prolonged, direct contect with active monkeypox lesions. In contrast, the quantities of virus isolated from the other six samples were all below the detectable limit of the assay, suggesting that even if an object is contaminated with monkeypox virus, the amount of virus is probably lower than the minimal infectious dose needed to establish a successful infection in a person.

A popular historical anecdote involving transmission of pox virus through inanimate objects describes how the US Army attempted to reduce the Native American population by gifting them smallpox virus-contaminated blankets. However, there are no reports or evidence indicating that this strategy actually worked, suggesting that it was probably ineffective. In fact, some historians cast doubt on whether this incident even took place.   

It is possible for pox viruses to remain viable on surfaces for long periods of time, but the conditions have to be just right. Studies involving variola virus (which causes smallpox) have shown that viral particles can stay viable on contaminated surfaces for up to 13 years if they are maintained at low humidity, low temperature, and in the absence of UV radiation. The authors of the Utah home study note that the residents cleaned and disinfected their home routinely during their illness, a practice that likely resulted in reduced viability and/or inactivation of infectious virus on all of the tested objects. In contrast, no such cleaning activities were noted for the Texas home. In addition, all windows of the Texas home were covered with closed blinds, a condition that likely reduced exposure to UV radiation and probably contributed to the preservation of viral particles.

Although it is possible to become infected with monkeypox virus by touching contaminated surfaces, the potential for this type of transmission appears to be limited, and the risk of acquiring monkeypox from hotel sheets, towels, or plane seats is probably very low. Monkeypox virus is transmitted most effectively through direct (i.e., skin-to-skin) contact with lesion material or inhalation of respiratory droplets during prolonged face-to-face interaction with an infected person.

Transmission of Enteric Viruses through Saliva

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by Gertrud U. Rey

Norovirus and rotavirus are considered to be enteric pathogens because they are traditionally thought to be transmitted by the fecal-oral route; i.e., when consuming food prepared by someone who did not wash their hands properly after using the bathroom. Unlike rabies virus, which replicates in the salivary glands and transmits through saliva, the presence of noroviruses and rotaviruses in saliva is usually considered to result from contamination, for example by vomitus or gastroesophageal reflux.

It was therefore surprising when a recent study showed that neonatal mice (“pups”) can apparently transmit enteric viruses to their mothers during suckling. Following oral inoculation with mouse norovirus 1 (MNV-1) or epizootic diarrhea of infant mice (EDIM, a mouse rotavirus), pups had secretory IgA (sIgA) in their intestines that increased gradually over the course of two weeks. sIgA is a type of antibody that provides immunity in mucous membranes such as those of the mouth, nose, and gut, and sIgA is typically passed from mothers to infants during suckling. Interestingly, this sIgA increase in pups correlated with a similar increase in the sIgA in the milk of the mothers, even though the mothers were not infected and did not have any antibodies to either virus at the start of the experiment (i.e., they were seronegative). Considering that pups can’t produce their own sIgA, it is likely that the pups infected the mothers during lactation, who then produced the sIgA and passed it back to the pups through their milk.

To rule out the possibility that the mothers became infected by consuming feces-contaminated food (i.e., by the traditional fecal-oral route) because they shared a living area with their infected pups, the authors orally inoculated pup-free seronegative mothers with EDIM, treated them with oxytocin to induce milk production, and analyzed the milk for sIgA and the mammary glands for viral RNA. Although the mothers were successfully infected, as evidenced by the presence of EDIM RNA in their small intestines, there was no detectable EDIM RNA in their mammary glands or detectable increase of sIgA in their milk. This result confirmed that the infected pups passed the virus to the mothers during feeding.

To further substantiate this finding, the authors orally infected one set of pups (pups A) with EDIM and placed them back in the cage with their seronegative mothers (mothers A) for suckling. The next day, mothers A were replaced with “mothers B” from a cage of uninfected pups (pups B), and mothers A were placed with pups B. Two days after this switch, all mice were euthanized and analyzed for the presence of viral RNA. All mothers had EDIM RNA in their mammary glands, suggesting that both sets of mothers became infected by suckling pups A. All pups had EDIM RNA in their small intestines, suggesting that pups B became infected by feeding from mothers A, or by ingesting their fecal matter.

The next set of experiments aimed to determine whether saliva contains enteric viruses and if it could serve as a means for transmission. Infection of adult mice with MNV-1 or EDIM revealed that these mice produced and shed virus in their saliva for up to ten days post infection. To see whether this saliva was infectious, the authors fed it to pups as a means of inoculation. At three days after infection, the pups had significantly high viral genome levels in their intestines, comparable to those observed in pups inoculated with virus of fecal origin. This result suggested that the viruses replicated in the intestines and confirmed that saliva can be a conduit for transmission of these enteric viruses.

In an effort to see whether these viruses replicate in the salivary glands, the authors orally infected pups and adults with various strains of norovirus and then isolated their submandibular glands – the largest component of the salivary gland complex. They then measured the number of infectious viral particles inside the submandibular glands using an alternative to the classical plaque assay known as a TCID50 assay, which quantifies the amount of virus needed to infect 50% of cells in culture. Each of the viruses increased in quantity by about 100,000-fold throughout the following three weeks compared to the input level, suggesting that noroviruses do replicate in the salivary glands. Treatment of mice with 2’-C-methylcytidine, an inhibitor of the norovirus polymerase enzyme, led to a decline of virus in the salivary glands, suggesting that this enzyme inhibited viral replication, and confirming that replication occurred inside the salivary glands.   

TCID50 analysis of various cell populations isolated from the submandibular gland revealed that MNV-1 replicates in epithelial and immune cells, both of which express Cd300lf, the gene encoding the intestinal receptor for all known strains of mouse norovirus. MNV-1 infection of mice lacking the Cd300lf gene led to no detectable MNV-1 replication in the salivary glands, suggesting that this receptor is needed for infection of submandibular gland cells. Partial extraction of the salivary glands from adult mice before inoculation led to faster clearing of the intestinal infection, suggesting that the salivary glands may serve as reservoirs for replication of these viruses.

The results of this study challenge the notion that noroviruses and rotaviruses transmit primarily by the fecal-oral route and raise several interesting questions. Do human noroviruses replicate in salivary glands, and do humans transmit noroviruses through saliva? If so, would protective measures in addition to handwashing (like face masks) prevent transmission of noroviruses? Are other enteric viruses (like poliovirus) also transmitted through saliva? It will be interesting to see future studies that address these questions.

[This paper was discussed in detail on TWiV 915.]

Should we be worried about monkeypox?

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by Gertrud U. Rey

The prevalence of monkeypox cases is continuing to increase around the world, with 7,243 total confirmed global cases as of today. Although this sounds awfully familiar, monkeypox virus is highly unlikely to cause a pandemic like the one we are presently experiencing, for at least two reasons: 1) monkeypox virus is not transmitted as easily as SARS-CoV-2, and 2) we have all the tools needed for quelling local outbreaks, thus hopefully preventing further community spread.

Because monkeypox has been endemic to Central and West Africa for several decades, scientists have had ample time to develop a thorough understanding of the virus and its associated disease. Monkeypox virus belongs to the Poxviridae, a family of viruses that also includes cowpox virus, variola virus (which causes smallpox), and vaccinia virus (the source of the modern smallpox vaccine). The name “monkeypox” resulted from the fact that the virus infects primates and was initially isolated from a laboratory monkey. However, it is actually thought to also circulate in rodents, which occasionally come into contact with humans, who can then further spread it to other humans.

Human-to-human transmission of monkeypox virus is far less efficient than that of SARS-CoV-2, which is commonly spread in the absence of symptoms, whereas monkeypox virus is only thought to be transmitted while an infected person is symptomatic. In addition, SARS-CoV-2 is readily spread when an infected person breathes, sneezes, or coughs around other people. In contrast, monkeypox virus is only transmitted by direct contact with lesion material or inhalation of respiratory droplets during prolonged face-to-face interaction with an infected person. Recent news reports have highlighted clusters of infections among men who have sex with men, leading some to infer that monkeypox is a sexually-transmitted disease. However, there is no evidence to suggest that the virus is present in sexual bodily fluids, therefore, it is not considered to be a sexually-transmitted pathogen. The high incidence of infections in the gay community could be explained by transmission through very close contact, which, by definition, includes sex.

The incubation period for monkeypox virus can range from 5 to 21 days, with an average of one week between infection and onset of symptoms. Initial symptoms usually include fever, swollen lymph nodes, headache, and muscle aches; and these symptoms are followed by a distinctive skin rash consisting of clear fluid-filled vesicles. The vesicles eventually fill with pus and ultimately crust over to give way to a new layer of healthy skin. Early symptoms are similar to those of chickenpox, which is caused by varicella-zoster virus (a herpesvirus, unrelated to poxviruses). However, unlike chickenpox lesions, which can individually exist in different stages of development throughout the course of infection, monkeypox lesions typically appear, progress, and disappear together.

Should the need arise, there are at least two licensed smallpox-specific vaccines that can also prevent monkeypox. ACAM2000 is a replication-competent live-attenuated vaccinia virus developed by Sanofi Pasteur Biologics Co. This vaccine is administered with a traditional bifurcated needle, and although very effective, it is associated with pretty severe side effects, including sore arm, fever, body aches, and occasional myocarditis. MVA-BN (marketed as “Jynneos” in the US) is a highly attenuated replication-incompetent vaccinia virus produced by Bavarian Nordic. MVA-BN/Jynneos is delivered by injection under the skin, is much better tolerated than ACAM2000, and is approved to be used as a monkeypox-specific vaccine. Fortunately, because of the long incubation period, it is possible to be vaccinated shortly after an exposure to monkeypox virus and still be protected from monkeypox disease.

It is unclear how long either of the available vaccines protect a person from disease, and whether individuals who were immunized against smallpox decades ago are protected from monkeypox today. Routine global smallpox vaccination ended in the late 1970s, so it is likely that the current outbreaks are fueled by non-immune people who were born since then, and/or by vaccinated individuals whose immunity has waned. However, even if infections continue to increase in number, it is unlikely that everybody in the general population would need to be vaccinated. Instead, proactively administering the vaccine to contacts and contacts of contacts of an infected person in a strategy termed “ring vaccination” would probably be sufficient to stop spread. That is, the vaccine would be administered in an area in a ring around the outbreak.

There are also several FDA-approved antiviral drugs that could be effective against monkeypox virus infection. Tecovirimat, which can be taken orally, prevents release of newly formed viral particles from infected cells, thus potentially blocking transmission of monkeypox virus. Cidofovir (administered by infusion into the vein) and its derivative brincidofovir (taken orally), disrupt replication of smallpox virus and could thus also be used for treating monkeypox virus infection.  

Considering all these factors, the average person is at low risk of becoming infected with monkeypox virus. Nevertheless, the World Health Organization has declared that there is no room for complacency and is urging governments to take some coordinated action to stop the spread of the virus. Because we have the tools to deal with monkeypox outbreaks and have hopefully learned from the disorganized manner in which the present pandemic was handled initially, a federal preparedness response should be implemented as soon as possible.

[The monkeypox outbreak was previously covered at least on Infectious Disease Puscast episodes 3 and 4; TWiV 902, TWiV 915; and TWiV Special Monkeypox Clinical Update with Dr. Daniel Griffin.]

Herd Immunity and this Pandemic

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by Gertrud U. Rey

Photo courtesy of Andrea Lightfoot photography

Herd immunity occurs when a large enough percentage of the population has acquired either natural or vaccine-induced immunity against an infectious disease, thereby indirectly protecting a minority of non-immune individuals who are dispersed throughout the population. During this pandemic, many prominent scientists have stated that it is impossible to achieve herd immunity in the context of COVID-19, leading some to conclude that a mass SARS-CoV-2 vaccination campaign would be pointless. However, this thinking is flawed, and I want to explain why.

Traditionally, herd immunity is thought to create a barrier for the transmission of infectious agents, resulting not only in prevention of disease, but also prevention of infection. This understanding was based on previous observations that vaccination against poliovirus, measles virus, and other pathogens led to drastic reductions in the incidence of disease burden. It is reasonable to assume that if there is no disease, there is probably also no virus; and hence no viral infection or transmission of virus. However, past vaccination campaigns were not followed up with regular testing programs, so we actually have no way of knowing whether vaccination prevented infection and transmission! Considering that the vast majority of poliovirus infections are asymptomatic, it is possible that some polio virus infections and transmission occurred even after vaccination, despite the fact that those infections did not lead to disease.

The widespread testing measures adopted during the present pandemic have revealed the approximate frequency of asymptomatic SARS-CoV-2 infections, giving us a clearer understanding of the difference and dynamics between disease and infection. The type of immunity that prevents both disease and infection is called sterilizing immunity, and it is mostly thought to be induced by neutralizing antibodies, which inactivate infectious agents before they have a chance to infect a cell, thereby directly neutralizing the biological effect of the agent. However, any immune activity that prevents replication of a pathogen directly or indirectly necessarily induces sterilizing immunity, including the activity of non-neutralizing antibodies, whose binding can trigger other immune functions that can also prevent infection and replication.

Do SARS-CoV-2 vaccines induce sterilizing immunity? The answer to this question is complicated. There are many studies showing that most people have high levels of antibodies in the months following vaccination, and this large proportion of circulating antibodies could likely sequester an incoming virus before it has a chance to enter cells, infect them, and replicate. In this sense, the SARS-CoV-2 vaccines do induce sterilizing immunity, but only within a certain time period after vaccination. As antibody levels contract over time (a normal process), they leave behind a baseline population of memory B cells that can quickly expand and mass-produce new antibodies upon a subsequent encounter with SARS-CoV-2. Likewise, memory T cells can quickly react to incoming virus and virus-triggered signals, and destroy infected cells. Therefore, it is likely that when circulating SARS-CoV-2-specific antibody levels decline months and years after vaccination, the collective activity of memory immune cells will protect one from disease, but probably not infection, meaning that the SARS-CoV-2 vaccines no longer induce sterilizing immunity at that time. In other words, vaccinated people could briefly replicate and transmit low levels of virus, at least until memory immune responses kick in, which then prevent illness and additional viral replication and spread.

The emergence of new variants that are not as well recognized by existing vaccine-induced antibodies may also allow for some increased viral transmission, thus slowing down the establishment of immunity in the population. However, immune responses are not binary, and even a low level immune response that doesn’t protect against infection and spread but prevents serious disease can play a critical role in slowing down the pandemic. Vaccination has historically been very effective at suppressing community outbreaks, despite the fact that most vaccines do not induce sterilizing immunity.

Vaccination or natural immunity do not have to prevent all infections, and immunity does not have to last a lifetime for a pandemic to end. Pediatrician and vaccinologist Paul Offit defines herd immunity as the point where the serious disease burden is reduced sufficiently so as to no longer overwhelm the healthcare system. It’s becoming pretty clear that the pandemic is slowing down in the US, especially in the context of severe disease, hospitalization, and death; and that this is likely due to increased SARS-CoV-2 immunity among US residents. It is therefore likely that reduced illness and a shortened period of transmission from immune individuals will also reduce the overall rate of community infection and transmission. And in the end, it doesn’t matter whether we call it herd immunity, community immunity or some other name; the pandemic will end because a majority of the population is no longer susceptible to severe COVID-19.

[Please check out my video Catch This Episode 29 for an explanation of sterilizing immunity.]

Estimate of infectiousness during COVID-19

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Understanding the transmission of SARS-CoV-2 is complicated by the large numbers of presymptomatic, asymptomatic, and mildly symptomatic (PAMS) patients. The reproductive number, R0, is a measure of population-level dynamics, but it cannot provide information on infectiousness of different groups such as PAMS subjects; when peak infectiousness occurs; and the effect of intrinsic properties of the virus. However information on infectiousness can be provided by a study of viral RNA load and infectivity in cell culture.

A study of 25,381 German COVID-19 cases was done to provide information on infectiousness during the course of infection. These included 6110 PAMS cases, 9159 hospitalized patients, and a series of times samples from hospitalized patients. Viral RNA loads – defined as RNA copies per swab – were determined by RT-PCR. The association between viral RNA load and probability of virus isolation in cell culture was estimated by using previously determined cell culture isolation data.

The results show that PAMS subjects have viral RNA loads (pictured) and predicted infectiousness, at the first positive test, slightly less than those of hospitalized patients. Children had similar viral RNA loads and predicted cell culture infectivity.

A small fraction of individuals’ first-positive viral RNA loads – 8%- had 10e9 RNA copies per swab. This observation is in line with previous findings that 15% of index cases are responsible for 80% of transmission, and that 8-9% of infected individuals carry 90% of the total viral load.

Time-course analysis revealed that viral RNA load peaks 1 to 3 days before onset of symptoms. From this peak, viral RNA loads decline 0.17 log10 units per day. Previous studies have shown that infectious virus is typically not recovered in cell culture beyond 10 days from symptom onset.

This study also examined viral RNA loads in 1533 patients with alpha variant infections. Compared with non-alpha infections, patients infected with the alpha variant had 10 times higher viral RNA loads and a 2.6 fold higher estimated cell culture infectivity. However, the impact of an increase in viral RNA load is, as the authors write, ‘dependent on context’ and the increase in cell culture infection probability is a ‘proxy indication of potentially higher transmissibility’. Complicating this analysis is the possibility that the correlation of viral RNA loads with cell culture infectivity might differ among variants.

The most important outcome of this study is that PAMS subjects – who in this study were tested at walk-in centers – are as infectious as hospitalized patients. As these individuals circulate in the community they can clearly trigger outbreaks of infection.

Holiday travel explains spread of a SARS-CoV-2 variant

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coronavirus Spike

The emergence and spread throughout Europe of a SARS-CoV-2 variant, 20E (EU1) in the summer of 2020 illustrates how a virus may become dominant not by increased transmissibility but through travel and lack of effective containment and screening.

The SARS-CoV-2 variant 20E (EU1) emerged in Spain in the summer of 2020 and spread to multiple European countries. By the fall of 2020 most of the sequences in Europe were from 20E (EU1). The variant bears a number of amino acid changes including A222V in the N-terminal domain of the spike protein (pictured).

Results of binding and neutralization assays revealed that the A222V change did not affect interaction of the spike protein with polyclonal or monoclonal antibodies. Lentivirus particles bearing the spike with A222V did not reproduce with higher efficiency in cells in culture. Authors conclude that these observations are not consistent with increased transmissibility of the 20E (EU1) variant. However, I would argue that studying the infectivity of a pseudotyped virus – in this case a lentivirus bearing the SARS-CoV-2 spike – in 293 cells (possibly human embryonic kidney) has virtually no relevance to what occurs in humans. At the very least, authentic SARS-CoV-2 infection of human respiratory tract cells must be examined. Even then, the results may have no direct bearing on what occurs in humans.

In contrast, epidemiological evidence explains the spread of 20E (EU1). This variant arose first in Spain in early summer 2020. It then spread extensively in Spain and also spread to other European countries. Results of modeling indicate that the spread of the variant can be explained by holiday travel-associated transmission (many EU countries opened their borders to travel on 15 June), and human behaviors such as failure to distance, mask, restrict gatherings, and adequately test for infections.

It seems likely that human behavior might also account for the global spread of other SARS-CoV-2 variants (e.g. alpha and beta). Authors believe that such spread is due to increased inherent transmission of the viruses, but the data in support of this conclusion are not convincing. Increased reproduction of pseudotyped viruses in cells in culture, as pointed out above, is likely irrelevant. Increased shedding of viral RNA from the nasopharynx, detected by PCR, is also irrelevant as it does not represent infectious virus. In no case has shedding of infectious virus from the nasopharyngeal tract been studied to address potential mechanisms of increased transmission. It is claimed that the reproductive index of variants is increased, but these calculations are flawed. The reproductive index is determined by a formula that includes both viral and host factors. However, host factors are never included when this index is determined for individual variants. As the study above indicates, human activities can substantially affect predominance of a virus in a population.

Why do variants out-compete and displace other viruses? It is because the variants have increased fitness, the ability of a virus to reproduce in the host. Fitness can be altered in many ways, including evasion of antibody responses, increased particle stability, and even person to person transmission. No experiments have been done to explain the increased fitness of variants. Fitness is not the same as transmission.

Variants of influenza virus arise frequently, and these variants displace existing viruses because they have a fitness advantage. For influenza virus, a fitness advantage is often conferred by HA amino acid changes that allow escape from antibody neutralization. Antigenic variants can infect a slightly larger number of hosts and that is enough natural selection advantage for the new variants to outcompete the older ones. No one ever says that these influenza virus variants have increased transmission.

Unfortunately the narrative that the variants have increased transmission is dominating the media. This situation has arisen because virologists, epidemiologists, and evolutionary biologists are not talking to one another. In addition, what we know about other viruses, as illustrated here for influenza virus, is also ignored.