As we move into the third year of this pandemic, it isn’t always easy to appreciate just how far we have come. But from a scientific perspective, the medical advancements are truly unparalleled, with the amazingly rapid development and implementation of high-performance diagnostic tests, vaccines, and novel antiviral medications that are reducing the human cost of Covid-19. In this blog, we take a look back and see how drugs, vaccines, and testing tools have advanced, and how they hold hope not only for our emergence from this pandemic but through a ‘lessons learned’ approach, examine how these technical and medical innovations could help lessen the impact of future epidemics.
Anti-Viral Drugs
In recent months there have been two additions to the medical arsenal against Covid-19, with the EUA approval of two anti-viral oral medications- Molnupiravir from Merck and Ridgeback Therapeutics and Paxlovid from Pfizer. These drugs are helping to keep Covid-19 patients out of the hospital and fill in the gap between the original antivirals such as monoclonal antibodies that must be given in the monitored setting of a physician’s office. With the new and unprecedented wave of the Omicron variant, these new therapeutics are game-changers in the way the medical field is managing the pandemic and preserving the lives of those most at risk.
Molnupiravir is a pro-drug of the synthetic nucleoside derivative N4-hydroxycytidine. This nucleoside derivative is incorporated in the viral RNA, creating errors in the virus as it multiplies to the point where the virus is no longer able to replicate, a process referred to as error catastrophe.
Paxlovid works differently, by inhibiting the proteolytic enzymatic process required for viral assembly, thereby preventing viral replication.
The coupling of these antiviral medications with anti-inflammatories is helping to prevent severe Covid by reducing the viral load and moderating the immune response to the infection. In clinical trials, both drugs were shown to reduce deaths and hospitalizations (50% with Molnupiravir and 85% with Paxlovid) but neither appeared to greatly improve the outcome of severe/hospitalized patients, presumably because the inflammatory response to the virus had already taken hold.
This pandemic has put a global focus on the need for novel antivirals. Although a wide range of antibiotics has been developed by biotech and pharma companies, anti-viral drugs remained elusive principally for two reasons- Biology and Economics.
Viruses use the host cells to replicate, and therefore killing viruses is easy, doing this without damaging our own cells is not. There are a few anti-viral successes- most notably Zanamivur and Tamiflu that are both used to treat patients with influenza and work by blocking a key viral enzyme involved in the release of the virus particles from the host cells. Acyclovir was one of the first widely used anti-viral drugs used to treat herpes simplex, chickenpox, and shingles by blocking DNA replication mediated by the viral (but not host) DNA polymerase. It has taken decades of investment and drug research to transform the treatment of patients with HIV/AIDS and Hepatitis C with targeted antivirals, but it has been a long and hard road to success.
The harsh truth is that until Covid-19, the economics of drugs targeting viral infections was difficult for the private sector to digest, and so significant public funding was required; this was highlighted in the case of drugs to manage HIV infection. For infections such as Zika and Ebola that are primarily impacting developing countries and that display very short therapeutic windows, costly antiviral drug approaches are shockingly still considered economically and/or logistically unfeasible.
Hopefully, Covid-19 has re-focused the industry’s antiviral drug development pipelines (at least for now) and opened the eyes of Venture Capital Investors, Government Agencies, and the Private Sector to the commercial value of establishing libraries of antiviral compounds for future screening and drug development initiatives.
Vaccines
By far the most impactful advances in addressing the pandemic have been in the development of highly effective vaccines. With 21 vaccines rolled out across the globe, and 200 in development (Ref. Gavi, The Vaccine Alliance- January 2022) as of January 2022, 58.8% of the world population has received at least one dose of Covid-19 vaccine; this equates to 9.33 billion doses (roughly 30.42 million doses each day currently). However, with only 8.8% of people in low-income countries receiving at least one dose, we have a long way to go. (daily updates available online at Coronavirus (COVID-19) Vaccinations - Statistics and Research - Our World in Data)
But not all Covid-19 vaccines are equivalent, and the reported efficacy rates vary between vaccine types and between different manufacturers.
Figure 1. Summary of the performance of the most widely adopted Covid-19 vaccines across the globe. It is important to note that each vaccine may have been assessed through clinical trials using different approaches to assess efficacy against the alpha variant- there is no data available on efficacy against different viral variants.
In addition, there are many factors that influence the effectiveness of a vaccine within an individual-in fact, every individual responds differently to a given vaccination protocol. In traditional vaccine clinical trials, years of data are collected to track vaccine performance over time, however, the expedited clinical trials for the Covid-19 vaccines didn’t permit this level of interrogation, and therefore serological surveillance programs will be critical to fill in this gap in our knowledge.
Vaccine efficacy is based on several factors summarized in Fig. 2. The recent pandemic has highlighted how much we still have a lot to learn about vaccine effectiveness and the full spectrum of immune systems mechanisms that are engaged by vaccination.
Figure 2. Vaccine Efficacy is a complex and somewhat unpredictable parameter. The Covid-19 pandemic has enabled the scientific community to evaluate the factors influencing vaccine responses as never before. Essentially, vaccine response is a truly personalized event- every one of us responds differently, in the immediate- short- and long-term responses. Understanding the critical drivers of vaccine efficacy will not only have implications for individuals but also for public health initiatives for generations to come.
The immune response to vaccination is complex and multifaceted, involving the engagement of both the adaptive and innate immune response pathways. A typical vaccine response profile begins with the production of Immunoglobulins (Ig) - IgM and IgA antibodies are generally detectable within 14 days after the initial vaccination. IgG then takes over as the primary immunoglobulin as IgM and IgA levels wane and continue the work of binding and neutralizing the pathogen. The so-called ‘neutralizing antibodies’ (NAb) represent those immunoglobulins that block the entry of the virus into the cell- in the case of SARS-CoV-2 this involves blocking the interaction between the RBD region of the Spike protein that peppers the surface of the virus and the ACE2 inhibitor on the surface of the cells. Several studies have correlated the levels of NAb with vaccine effectiveness (Gilbert, P. B. et. al. 2021) and with the establishment of increased levels of SARS-CoV-2 specific T- and NK- cells (Pan, Y. et. al. 2021)
Robust and effective antibody responses to infection or vaccination hinge on two different facets: the levels or titers of antibodies raised to the target antigens of the vaccine, and specifically, the titers of the critical neutralizing antibodies. There are various methods that have been used to quantify immune responses to vaccination including measuring Geometric Mean Titers/Concentrations (GMT/GMCs), SeroConversion Rates (SCRs), SeroProtection Rates (SPRs), functional assays (typically performed using flow cytometric opsonophagocytosis assays), antibody avidity, the strength of cell-mediated immune responses (B-cell and T-cell activation and memory) and cytokine responses. There is a great deal of variation reported for different vaccines with all of these parameters, that collectively influence the level and duration of protective efficacy.
Figure 3. Summary of Approaches and Methods Used to Quantify Vaccine Responses. The complexity of responses is underpinned by distinct factors and pathways that collectively contribute to vaccine immunogenicity effectiveness.
With such diversity in responses, there is certainly value in assessing vaccine responses at both the population and individual levels. And as boosters are now widely available across North America and Europe, the ability to optimally determine when an individual should be receiving a booster vaccine may help support responsible use of these vaccines and support the stewardship of global vaccine supplies.
So how where does Serology Testing fit in to all of this?
Serology Testing monitors for the presence of antibodies that target the virus, and by comparing antibody levels targeting proteins both represented in the vaccine, and those only present on the virus, serology testing can be used to identify past infections as well as look at vaccine efficacy profiles. Unlike PCR and antigen screening assays, serology tests are not diagnostic, but rather can be used for population profiling and epidemiology studies (Espejo et. al. 2020).
There are caveats with serology testing- timing is everything and testing too soon after infection or vaccination could result in a false negative since there may not be a detectable level of antibody generated at that point in time. Too long post-infection or vaccination and antibody levels may have waned below detectable levels. In addition, since SARS-CoV-2 isn’t the only coronavirus, it is possible that a poorly engineered test will detect antibodies to some of the common coronaviruses such as types 229E, NL63, OC43, and HKU1 that are responsible for causing the common cold - and provide a false positive.
As with most clinical tests, the serology test is a balance between sensitivity and specificity.
Sensitivity represents the ability to detect individuals with anti-SARS-CoV-2 antibodies i.e., the true positivity rate.
Specificity represents the test’s ability to correctly identify people without anti-SARS-CoV-2 antibodies- i.e., the true negativity rate.
From these profiles we can assess an assay's performance by calculating the following:
Positive Predictive Value- is defined as the probability that people who have a positive test truly have anti-SARS-CoV-2 antibodies.
Negative Predictive Value is the probability that people who have a negative test, truly have no detectable anti-SARS-CoV-2 antibodies.
The concept of predictive values can be a little challenging since they are calculated based on a test’s sensitivity and specificity, coupled with an assumption about the percentage of the population who have anti-SARS-CoV-2 at any given time (also known as the prevalence). Obviously, the prevalence of anti-SARS-CoV-2 antibodies within the population has changed markedly over the course of this pandemic.
Over the past 18 months, FlowMetric has developed and validated a powerful and novel serology test that couples the detection of two endpoints (anti-RBD and NC) to distinguish between vaccine response and the immune response to natural infectionn. High complexity serology assays provide higher positive predictive values than single dimension ELISAs or Lateral Flow serology tests and not only can help individuals assess their response to vaccination, but also identify if an individual has been previously infected with SARS-CoV-2, perhaps asymptomatically. FlowMetric’s flow cytometry-based serology test also delivers an enhanced dynamic range of detection of antibody titers to quantitatively measure early and long-term immune responses to vaccination and infection. Although currently there are no FDA or CDC guidelines that correlate anti-SARS-CoV-2 antibody titers to immunity, it is important to note that antibody responses were regarded as clinical endpoints within all of the Covid-19 vaccine clinical trials as an indicator or seroconversion. FlowMetric’s multiplexed serology test may offer value in sero-surveillance of the Covid-19 pandemic and evaluate the long-term protection afforded by the different vaccine types that are currently being utilized globally.
Value of Serology Testing for Population Surveillance
The CDC is closely working with local- state and commercial partners to use serology testing data to model the spread of Covid-19 across the USA, by region, and by population demographic. These so-called seroprevalence surveys can provide a wealth of information from the total number of people infected throughout the pandemic, to an assessment of the number of cases missed by diagnostic testing, as well as the identification of demographics most prone to infection and those most resistant to testing and vaccination.
Concurrently with the wave of the Omicron variant has been increased access to over-the-counter rapid testing for Covid-19. Self-administered rapid antigen tests involve a simple sample collection using a nose swab, that is placed into a solution that disrupts the virus particles. The solution is applied to a test strip that is coated with antibodies specific to SARS-CoV-2 that are printed in a line across the test strip. If a virus is present, then a colored line appears on the strip. The CDC recommends that unvaccinated individuals use these rapid tests as soon as possible after any possible exposure, or if experiencing symptoms. For vaccinated individuals, the CDC guidelines suggest waiting 5-7 days post-exposure. Serial testing over several days is highly recommended since it is possible to test negative during the early stages of infection. Studies have shown that when used this way, these rapid tests can be as effective at detecting infection as the gold standard RT-PCR tests that require processing within a lab.
The increased access and use of home rapid tests is a double-edged sword- it enables individuals to self-test more frequently and lowers the barrier to effective monitoring following exposure or for those working in high-risk environments. However, the use of these tests also means that a significant number of Covid-19 cases may not be reported to the CDC. As a result, serology testing may represent the only reliable way to track infection rates within populations and support essential epidemiological studies evaluating vaccine effectiveness over time with seropositive results during post-market surveillance.
Final Thoughts
Covid-19 has fueled unprecedented advances in medicine and reshaped many of our public health initiatives from expedited clinical vaccine trials to the implementation of the EUA approval process for drugs and diagnostics. Beyond these, there have also been significant advancements in digital technologies for contact tracing and quarantine enforcement that have certainly worked to help flatten the curve of infection. This has been coupled with a meteoric rise in telemedicine that has changed all of our lives. Defining long-term immunity to SARS-CoV-2 following infection or vaccination is an important goal that unfortunately hasn’t been clearly defined or communicated yet by the CDC or vaccine manufacturers. With the emergence of variants such as Delta and Omicron, it is likely that an annual Covid-19 vaccine may be the preferred path forward Pfizer CEO sees annual COVID vaccine rather than frequent boosters | Reuters
Governments and policymakers now have a unique opportunity to assess the repercussions of many of these technology drivers and flush out the policies and clinical tools that work for the greater good and that, if needed, will better equip us to address any future pandemic.
References
- Nygård M et al. (2018) Evaluation of the Long-Term Anti-Human Papillomavirus 6 (HPV6), 11, 16, and 18 Immune Responses Generated by the Quadrivalent HPV Vaccine. Clin Vaccine Immunol. 2015 Aug;22(8):943-8. DOI: 10.1128/CVI.00133-15
- Gilbert, P. B. et. al. (2021) Immune Assays Team; Moderna, Inc. Team; Coronavirus Vaccine Prevention Network (CoVPN)/Coronavirus Efficacy (COVE) Team; United States Government (USG)/CoVPN Biostatistics Team. Science. Nov 23: eab3435. Online ahead of print. PMID: 34812653.
- Pan, Y. et. al. (2021) SARS-CoV-2-specific immune response in COVID-19 convalescent individuals. Signal Transduction and Targeted Therapy volume 6, Article number: 256.
- Espejo, A. P. et al. (2020) Review of Current Advances in Serologic Testing for COVID-19. AJCP Review Article Am J Clin Pathol 154:293-304. DOI: 10.1093/AJCP/AQAA112
