Dr. Francis Collins
Discussing the Need for Reliable Antibody Testing for COVID-19
Posted on by Dr. Francis Collins
There’s been a great deal of discussion about whether people who recover from coronavirus disease 2019 (COVID-19), have neutralizing antibodies in their bloodstream to guard against another infection. Lots of interesting data continue to emerge, including a recent preprint from researchers at Sherman Abrams Laboratory, Brooklyn, NY [1]. They tested 11,092 people for antibodies in May at a local urgent care facility and found nearly half had long-lasting IgG antibodies, a sign of exposure to the novel coronavirus SARS-CoV-2, the cause of COVID-19. The researchers also found a direct correlation between the severity of a person’s symptoms and their levels of IgG antibodies.
This study and others remind us of just how essential antibody tests will be going forward to learn more about this challenging pandemic. These assays must have high sensitivity and specificity, meaning there would be few false negatives and false positives, to tell us more about a person’s exposure to SARS-CoV-2. While there are some good tests out there, not all are equally reliable.
Recently, I had a chance to discuss COVID-19 antibody tests, also called serology tests, with Dr. Norman “Ned” Sharpless, Director of NIH’s National Cancer Institute (NCI). Among his many talents, Dr. Sharpless is an expert on antibody testing for COVID-19. You might wonder how NCI got involved in COVID-19 testing. Well, you’re going to find out. Our conversation took place while videoconferencing, with him connecting from North Carolina and me linking in from my home in Maryland. Here’s a condensed transcript of our chat:
Collins: Ned, thanks for joining me. Maybe we should start with the basics. What are antibodies anyway?
Sharpless: Antibodies are proteins that your body makes as part of the learned immune system. It’s the immunity that responds to a bacterium or a virus. In general, if you draw someone’s blood after an infection and test it for the presence of these antibodies, you can often know whether they’ve been infected. Antibodies can hang around for quite a while. How long exactly is a topic of great interest, especially in terms of the COVID-19 pandemic. But we think most people infected with coronavirus will make antibodies at a reasonably high level, or titer, in their peripheral blood within a couple of weeks of the infection.
Collins: What do antibodies tell us about exposure to a virus?
Sharpless: A lot of people with coronavirus are infected without ever knowing it. You can use these antibody assays to try and tell how many people in an area have been infected, that is, you can do a so-called seroprevalence survey.
You could also potentially use these antibody assays to predict someone’s resistance to future infection. If you cleared the infection and established immunity to it, you might be resistant to future infection. That might be very useful information. Maybe you could make a decision about how to go out in the community. So, that part is of intense interest as well, although less scientifically sound at the moment.
Collins: I have a 3D-printed model of SARS-CoV-2 on my desk. It’s sort of a spherical virus that has spike proteins on its surface. Do the antibodies interact with the virus in some specific ways?
Sharpless: Yes, antibodies are shaped like the letter Y. They have two binding domains at the head of each Y that will recognize something about the virus. We find antibodies in the peripheral blood that recognize either the virus nucleocapsid, which is the structural protein on the inside; or the spikes, which stick out and give coronavirus its name. We know now that about 99 percent of people who get infected with the virus will develop antibodies eventually. Most of those antibodies that you can detect to the spike proteins will be neutralizing, which means they can kill the virus in a laboratory experiment. We know from other viruses that, generally, having neutralizing antibodies is a promising sign if you want to be immune to that virus in the future.
Collins: Are COVID-19 antibodies protective? Are there reports of people who’ve gotten better, but then were re-exposed and got sick again?
Sharpless: It’s controversial. People can shed the virus’s nucleic acid [genetic material], for weeks or even more than a month after they get better. So, if they have another nucleic acid test it could be positive, even though they feel better. Often, those people aren’t making a lot of live virus, so it may be that they never stopped shedding the virus. Or it may be that they got re-infected. It’s hard to understand what that means exactly. If you think about how many people worldwide have had COVID-19, the number of legitimate possible reinfection cases is in the order of a handful. So, it’s a pretty rare event, if it happens at all.
Collins: For somebody who does have the antibodies, who apparently was previously infected, do they need to stop worrying about getting exposed? Can they can do whatever they want and stop worrying about distancing and wearing masks?
Sharpless: No, not yet. To use antibodies to predict who’s likely to be immune, you’ve got to know two things.
First: can the tests actually measure antibodies reliably? I think there are assays available to the public that are sufficiently good for asking this question, with an important caveat. If you’re trying to detect something that’s really rare in a population, then any test is going to have limitations. But if you’re trying to detect something that’s more common, as the virus was during the recent outbreak in Manhattan, I think the tests are up to the task.
Second: does the appearance of an antibody in the peripheral blood mean that you’re actually immune or you’re just less likely to get the virus? We don’t know the answer to that yet.
Collins: Let’s be optimistic, because it sounds like there’s some evidence to support the idea that people who develop these antibodies are protected against infection. It also sounds like the tests, at least some of them, are pretty good. But if there is protection, how long would you expect it to last? Is this one of those things where you’re all set for life? Or is this going to be something where somebody’s had it and might get it again two or three years from now, because the immunity faded away?
Sharpless: Since we have no direct experience with this virus over time, it’s hard to answer. The potential for this cell-based humoral immunity to last for a while is there. For some viruses, you have a long-lasting antibody protection after infection; for other viruses, not so much.
So that’s the unknown thing. Is immunity going to last for a while? Of course, if one were to bring up the topic of vaccines, that’s very important to know, because you would want to know how often one would have to give that vaccine, even under optimal circumstances.
Collins: Yes, our conversation about immunity is really relevant to the vaccines we’re trying to develop right now. Will these vaccines be protective for long periods of time? We sure hope so, but we’ve got to look carefully at the issue. Let’s come back, though, to the actual performance of the tests. The NCI has been right in the middle of trying to do this kind of validation. How did that happen, and how did that experience go?
Sharpless: Yes, I think one might ask: why is the National Cancer Institute testing antibody kits for the FDA? It is unusual, but certainly not unheard of, for NCI to take up problems like this during a time of a national emergency. During the HIV era, NCI scientists, along with others, identified the virus and did one of the first successful compound screens to find the drug AZT, one of the first effective anti-HIV therapies.
NCI’s Frederick National Lab also has a really good serology lab that had been predominantly working on human papillomavirus (HPV). When the need arose for serologic testing a few months ago, we pivoted that lab to a coronavirus serology lab. It took us a little while, but eventually we rounded up everything you needed to create positive and negative reference panels for antibody testing.
At that time, the FDA had about 200 manufacturers making serology tests that hoped for approval to sell. The FDA wanted some performance testing of those assays by a dispassionate third party. The Frederick National Lab seemed like the ideal place, and the manufacturers started sending us kits. I think we’ve probably tested on the order of 20 so far. We give those data back to the FDA for regulatory decision making. They’re putting all the data online.
Collins: How did it look? Are these all good tests or were there some clunkers?
Sharpless: There were some clunkers. But we were pleased to see that some of the tests appear to be really good, both in our hands and those of other groups, and have been used in thousands of patients.
There are a few tests that have sensitivities that are pretty high and specificities well over 99 percent. The Roche assay has a 99.8 percent specificity claimed on thousands of patients, and for the Mt. Sinai assay developed and tested by our academic collaborators in a panel of maybe 4,000 patients, they’re not sure they’ve ever had a false positive. So, there are some assays out there that are good.
Collins: There’s been talk about how there will soon be monoclonal antibodies directed against SARS-CoV-2. How are those derived?
Sharpless: They’re picked, generally, for appearing to have neutralizing activity. When a person makes antibodies, they don’t make one antibody to a pathogen. They make a whole family of them. And those can be individually isolated, so you can know which antibodies made by a convalescent individual really have virus-neutralizing capacity. That portion of the antibody that recognizes the virus can be engineered into a manufacturing platform to make monoclonal antibodies. Monoclonal means one kind of antibody. That approach has worked for other infectious diseases and is an interesting idea here too.
Collins: I can say a bit about that, because we are engaged in a partnership with industry and FDA called Accelerating COVID-19 Therapeutic Interventions and Vaccines (ACTIV). One of the hottest ideas right now is monoclonal antibodies, and we’re in the process of devising a master protocol, one for outpatients and one for inpatients.
Janet Woodcock of Operation Warp Speed tells me 21 companies are developing monoclonal antibodies. While doing these trials, we’d love to do comparisons, which is why it’s good to have an organization like ACTIV to bring everybody together, making sure you’re using the same endpoints and the same laboratory measures. I think that, maybe even by late summer, we might have some results. For people who are looking at what’s the next most-hopeful therapeutic option for people who are really sick with COVID-19, so far we have remdesivir. It helps, but it’s not a home run. Maybe monoclonal antibodies will be the next thing that really gives a big boost in survival. That would be the hope.
Ned, let me ask you one final question about herd, or group, immunity. One hears a bit about that in terms of how we are all going to get past this COVID-19 pandemic. What’s that all about?
Sharpless: Herd immunity is when a significant portion of the population is immune to a pathogen, then that pathogen will die out in the population. There just aren’t enough susceptible people left to infect. What the threshold is for herd immunity depends on how infectious the virus is. For a highly infectious virus, like measles, maybe up to 90 percent of the population must be immune to get herd immunity. Whereas for other less-infectious viruses, it may only be 50 percent of the population that needs to be immune to get herd immunity. It’s a theoretical thing that makes some assumptions, such as that everybody’s health status is the same and the population mixes perfectly every day. Neither of those are true.
How well that actual predictive number will work for coronavirus is unknown. The other thing that’s interesting is a lot of that work has been based on vaccines, such as what percentage do you have to vaccinate to get herd immunity? But if you get to herd immunity by having people get infected, so-called natural herd immunity, that may be different. You would imagine the most susceptible people get infected soonest, and so the heterogeneity of the population might change the threshold calculation.
The short answer is nobody wants to find out. No one wants to get to herd immunity for COVID-19 through natural herd immunity. The way you’d like to get there is with a vaccine that you then could apply to a large portion of the population, and have them acquire immunity in a more safe and controlled manner. Should we have an efficacious vaccine, this question will loom large: how many people do we need to vaccinate to really try and protect vulnerable populations?
Collins: That’s going to be a really critical question for the coming months, as the first large-scale vaccine trials get underway in July, and we start to see how they work and how successful and safe they are. But I’m also worried seeing some reports that 1 out of 5 Americans say they wouldn’t take a vaccine. It would be truly a tragedy if we have a safe and effective vaccine, but we don’t get enough uptake to achieve herd immunity. So, we’ve got some work to do on all fronts, that’s for sure.
Ned, I want to thank you for sharing all this information about antibodies and serologies and other things, as well as thank you for your hard work with all your amazing NCI colleagues.
Sharpless: Thanks for having me.
Reference:
[1] SARS-CoV-2 IgG Antibody Responses in New York City. Reifer J, Hayum N, Heszkel B, Klagsbald I, Streva VA. medRxiv. Preprint posted May 26, 2020.
Links:
Coronavirus (COVID-19) (NIH)
At NCI, A Robust and Rapid Response to the COVID-19 Pandemic. Norman E. Sharpless. Cancer Currents Blog. April 17, 2020 (National Cancer Institute/NIH)
Serological Testing for SARS-CoV-2 Antibodies (American Medical Association, Chicago)
COVID-19 Antibody Testing Primer (Infectious Diseases Society of America, Arlington, VA)
Accelerating COVID-19 Therapeutic Interventions and Vaccines (NIH)
Will Warm Weather Slow Spread of Novel Coronavirus?
Posted on by Dr. Francis Collins
With the start of summer coming soon, many are hopeful that the warmer weather will slow the spread of SARS-CoV-2, the novel coronavirus that causes COVID-19. There have been hints from lab experiments that increased temperature and humidity may reduce the viability of SARS-CoV-2. Meanwhile, other coronaviruses that cause less severe diseases, such as the common cold, do spread more slowly among people during the summer.
We’ll obviously have to wait a few months to get the data. But for now, many researchers have their doubts that the COVID-19 pandemic will enter a needed summertime lull. Among them are some experts on infectious disease transmission and climate modeling, who ran a series of sophisticated computer simulations of how the virus will likely spread over the coming months [1]. This research team found that humans’ current lack of immunity to SARS-CoV-2—not the weather—will likely be a primary factor driving the continued, rapid spread of the novel coronavirus this summer and into the fall.
These sobering predictions, published recently in the journal Science, come from studies led by Rachel Baker and Bryan Grenfell at Princeton Environmental Institute, Princeton, NJ. The Grenfell lab has long studied the dynamics of infectious illnesses, including seasonal influenza and respiratory syncytial virus (RSV). Last year, they published one of the first studies to look at how our warming climate might influence those dynamics in the coming years [2].
Those earlier studies focused on well-known human infectious diseases. Less clear is how seasonal variations in the weather might modulate the spread of a new virus that the vast majority of people and their immune systems have yet to encounter.
In the new study, the researchers developed a mathematical model to simulate how seasonal changes in temperature might influence the trajectory of COVID-19 in cities around the world. Of course, because the virus emerged on the scene only recently, we don’t know very much about how it will respond to warming conditions. So, the researchers ran three different scenarios based on what’s known about the role of climate in the spread of other viruses, including two coronaviruses, called OC43 and HKU1, that are known to cause common colds in people.
In all three scenarios, their models showed that climate only would become an important seasonal factor in controlling COVID-19 once a large proportion of people within a given community are immune or resistant to infection. In fact, the team found that, even if one assumes that SARS-CoV-2 is as sensitive to climate as other seasonal viruses, summer heat still would not be enough of a mitigator right now to slow its initial, rapid spread through the human population. That’s also clear from the rapid spread of COVID-19 that’s currently occurring in Brazil, Ecuador, and some other tropical nations.
Over the longer term, as more people develop immunity, the researchers suggest that COVID-19 may likely fall into a seasonal pattern similar to those seen with diseases caused by other coronaviruses. Long before then, NIH is working intensively with partners from all sectors to make sure that safe, effective treatments and vaccines will be available to help prevent the tragic, heavy loss of life that we’re seeing now.
Of course, climate is just one key factor to consider in evaluating the course of this disease. And, there is a glimmer of hope in one of the group’s models. The researchers incorporated the effects of control measures, such as physical distancing, with climate. It appears from this model that such measures, in combination with warm temperatures, actually might combine well to help slow the spread of this devastating virus. It’s a reminder that physical distancing will remain our best weapon into the summer to slow or prevent the spread of COVID-19. So, keep wearing those masks and staying 6 feet or more apart!
References:
[1] Susceptible supply limits the role of climate in the early SARS-CoV-2 pandemic. Baker RE, Yang W, Vecchi GA, Metcalf CJE, Grenfell BT. Science. 2020 May 18. [Online ahead of print.]
[2] Epidemic dynamics of respiratory syncytial virus in current and future climates. Baker RE, Mahmud AS, Wagner CE, Yang W, Pitzer VE, Viboud C, Vecchi GA, Metcalf CJE, Grenfell BT.Nat Commun. 2019 Dec 4;10(1):5512.
Links:
Coronavirus (COVID-19) (NIH)
Bryan Grenfell (Princeton University, Princeton, NJ)
Rachel Baker (Princeton University, Princeton, NJ)
A Novel, 3D-Printed Virus
Posted on by Dr. Francis Collins
In the blog post “3D Printing the Novel Coronavirus,” I mentioned receiving my very own 3D-printed model of the novel coronavirus SARS-CoV-2, the cause of COVD-19. I shared this impressive model, created by the NIH 3D Print Exchange, with viewers during our third virtual NIH Town Hall meeting. The one-hour meeting was held on May 21, 2020 for employees, contractors, fellows, and trainees now sheltering at home to discuss the multi-phased return to the NIH workplace that will begin in June. Credit: NIH
3D Printing the Novel Coronavirus
Posted on by Dr. Francis Collins
The coronavirus disease 2019 (COVID-19) pandemic has truly been an all-hands-on-deck moment for the nation. Among the responders are many with NIH affiliations, who are lending their expertise to deploy new and emerging technologies to address myriad research challenges. That’s certainly the case for the dedicated team from the National Institute of Allergy and Infectious Diseases (NIAID) at the NIH 3D Print Exchange (3DPX), Rockville, MD.
A remarkable example of the team’s work is this 3D-printed physical model of SARS-CoV-2, the novel coronavirus that causes COVID-19. This model shows the viral surface (blue) and the spike proteins studded proportionally to the right size and shape. These proteins are essential for SARS-CoV-2 to attach to human cells and infect them. Here, the spike proteins are represented in their open, active form (orange) that’s capable of attaching to a human cell, as well as in their closed, inactive form (red).
The model is about 5 inches in diameter. It takes more than 5 hours to print using an “ink” of thin layers of a gypsum plaster-based powder fused with a colored binder solution. When completed, the plaster model is coated in epoxy for strength and a glossy, ceramic-like finish. For these models, NIAID uses commercial-grade, full-color 3D printers. However, the same 3D files can be used in any type of 3D printer, including “desktop” models available on the consumer market.
Darrell Hurt and Meghan McCarthy lead the 3DPX team. Kristen Browne, Phil Cruz, and Victor Starr Kramer, the team members who helped to produce this remarkable model, created it as part of a collaboration with the imaging team at NIAID’s Rocky Mountain Laboratories (RML), Hamilton, MT.
The RML’s Electron Microscopy Unit captured the microscopic 3D images of the virus, which was cultured from one of the first COVID-19 patients in the country. The unit handed off these and other data to its in-house visual specialist to convert into a preliminary 3D model. The model was then forwarded to the 3DPX team in Maryland to colorize and optimize in preparation for 3D printing.
This model is especially unique because it’s based exclusively on SARS-CoV-2 data. For example, the model is assembled from data showing that the virus is frequently oval, not perfectly round. The spike proteins also aren’t evenly spaced, but pop up more randomly from the surface. Another nice feature of 3D printing is the models can be constantly updated to incorporate the latest structural discoveries.
That’s why 3D models are such an excellent teaching tools to share among scientists and the public. Folks can hold the plaster virus and closely examine its structure. In fact, the team recently printed out a model and delivered it to me for exactly this educational purpose.
In addition to this complete model, the researchers also are populating the online 3D print exchange with atomic-level structures of the various SARS-CoV-2 proteins that have been deposited by researchers around the world into protein and electron microscopy databanks. The number of these structures and plans currently stands at well over 100—and counting.
As impressive as this modeling work is, 3DPX has found yet another essential way to aid in the COVID-19 fight. In March, the Food and Drug Administration (FDA) announced a public-private partnership with the NIH 3D Print Exchange, Department of Veterans Affairs (VA) Innovation Ecosystem, and the non-profit America Makes, Youngstown, OH [1]. The partnership will develop a curated collection of designs for 3D-printable personal protective equipment (PPE), as well as other necessary medical devices that are in short supply due to the COVID-19 pandemic.
You can explore the partnership’s growing collection of COVID-19-related medical supplies online. And, if you happen to have a 3D printer handy, you could even try making them for yourself.
Reference:
[1] FDA Efforts to Connect Manufacturers and Health Care Entities: The FDA, Department of Veterans Affairs, National Institutes of Health, and America Makes Form a COVID-19 response Public-Private Partnership (Food and Drug Administration)
Links:
Coronavirus (COVID-19) (NIH)
NIH 3D Print Exchange (National Institute of Allergy and Infectious Diseases/NIH, Rockville, MD)
Rocky Mountain Laboratories (NIAID/NIH, Hamilton, MT)
Department of Veterans Affairs (VA) Innovation Ecosystem (Washington, D.C.)
America Makes (Youngstown, OH)
NIH Support: National Institute of Allergy and Infectious Diseases
Enlisting Monoclonal Antibodies in the Fight Against COVID-19
Posted on by Dr. Francis Collins
We now know that the immune system of nearly everyone who recovers from COVID-19 produces antibodies against SARS-CoV-2, the novel coronavirus that causes this easily transmitted respiratory disease [1]. The presence of such antibodies has spurred hope that people exposed to SARS-CoV-2 may be protected, at least for a time, from getting COVID-19 again. But, in this post, I want to examine another potential use of antibodies: their promise for being developed as therapeutics for people who are sick with COVID-19.
In a recent paper in the journal Science, researchers used blood drawn from a COVID-19 survivor to identify a pair of previously unknown antibodies that specifically block SARS-CoV-2 from attaching to human cells [2]. Because each antibody locks onto a slightly different place on SARS-CoV-2, the vision is to use these antibodies in combination to block the virus from entering cells, thereby curbing COVID-19’s destructive spread throughout the lungs and other parts of the body.
The research team, led by Yan Wu, Capital Medical University, Beijing, first isolated the pair of antibodies in the laboratory, starting with white blood cells from the patient. They were then able to produce many identical copies of each antibody, referred to as monoclonal antibodies. Next, these monoclonal antibodies were simultaneously infused into a mouse model that had been infected with SARS-CoV-2. Just one infusion of this combination antibody therapy lowered the amount of viral genetic material in the animals’ lungs by as much as 30 percent compared to the amount in untreated animals.
Monoclonal antibodies are currently used to treat a variety of conditions, including asthma, cancer, Crohn’s disease, and rheumatoid arthritis. One advantage of this class of therapeutics is that the timelines for their development, testing, and approval are typically shorter than those for drugs made of chemical compounds, called small molecules. Because of these and other factors, many experts think antibody-based therapies may offer one of the best near-term options for developing safe, effective treatments for COVID-19.
So, what exactly led up to this latest scientific achievement? The researchers started out with a snippet of SARS-CoV-2’s receptor binding domain (RBD), a vital part of the spike protein that protrudes from the virus’s surface and serves to dock the virus onto an ACE2 receptor on a human cell. In laboratory experiments, the researchers used the RBD snippet as “bait” to attract antibody-producing B cells in a blood sample obtained from the COVID-19 survivor. Altogether, the researchers identified four unique antibodies, but two, which they called B38 and H4, displayed a synergistic action in binding to the RBD that made them stand out for purposes of therapeutic development and further testing.
To complement their lab and animal experiments, the researchers used a particle accelerator called a synchrotron to map, at near-atomic resolution, the way in which the B38 antibody locks onto its viral target. This structural information helps to clarify the precise biochemistry of the complex interaction between SARS-CoV-2 and the antibody, providing a much-needed guide for the rational design of targeted drugs and vaccines. While more research is needed before this or other monoclonal antibody therapies can be used in humans suffering from COVID-19, the new work represents yet another example of how basic science is expanding fundamental knowledge to advance therapeutic discovery for a wide range of health concerns.
Meanwhile, there’s been other impressive recent progress towards the development of monoclonal antibody therapies for COVID-19. In work described in the journal Nature, an international research team started with a set of neutralizing antibodies previously identified in a blood sample from a person who’d recovered from a different coronavirus-caused disease, called severe acute respiratory syndrome (SARS), in 2003 [3]. Through laboratory and structural imaging studies, the researchers found that one of these antibodies, called S309, proved particularly effective at neutralizing the coronavirus that causes COVID-19, SARS-CoV-2, because of its potent ability to target the spike protein that enables the virus to enter cells. The team, which includes NIH grantees David Veesler, University of Washington, Seattle, and Davide Corti, Humabs Biomed, a subsidiary of Vir Biotechnology, has indicated that S309 is already on an accelerated development path toward clinical trials.
In the U.S. and Europe, the Accelerating COVID-19 Therapeutic Interventions and Vaccines (ACTIV) partnership, which has brought together public and private sector COVID-19 therapeutic and vaccine efforts, is intensely pursuing the development and testing of therapeutic monoclonal antibodies for COVID-19 [4]. Stay tuned for more information about these potentially significant advances in the next few months.
References:
[1] Humoral immune response and prolonged PCR positivity in a cohort of 1343 SARS-CoV 2 patients in the New York City region. Wajnberg A , Mansour M, Leven E, Bouvier NM, Patel G, Firpo A, Mendu R, Jhang J, Arinsburg S, Gitman M, Houldsworth J, Baine I, Simon V, Aberg J, Krammer F, Reich D, Cordon-Cardo C. medRxiv. Preprint Posted May 5, 2020.
[2] A noncompeting pair of human neutralizing antibodies block COVID-19 virus binding to its receptor ACE2. Wu Y. et al., Science. 13 May 2020 [Epub ahead of publication]
[3] Cross-neutralization of SARS-CoV-2 by a human monoclonal SARS-CoV antibody. Pinto D, Park YJ, Beltramello M, Veesler D, Cortil D, et al. Nature. 18 May 2020 [Epub ahead of print]
[4] Accelerating COVID-19 therapeutic interventions and vaccines (ACTIV): An unprecedented partnership for unprecedented times. Collins FS, Stoffels P. JAMA. 2020 May 18.
Links:
Coronavirus (COVID-19) (NIH)
Monoclonal Antibodies (National Cancer Institute/NIH)
Accelerating COVID-19 Therapeutic Interventions and Vaccines (ACTIV)
NIH Support: National Institute of Allergy and Infectious Diseases; National Institute of General Medical Sciences
Next Page
