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February 17, 2021 January 30 – February 17, 2021 This weekly science review is a snapshot of the new and emerging scientific evidence related to COVID-19 during the period specified. It is a review of important topics and articles, not a guide for policy or program implementation. The findings captured are subject to change as new information is made available. We welcome comments and feedback at [email protected]. In depth: Should second doses of COVID-19 vaccines be delayed? Main message: While the rollout of multiple vaccines has offered hope to control or end the COVID-19 pandemic, global vaccine demand will continue to outpace supply for the foreseeable future. To date, people in 130 countries have not yet received a single dose of COVID-19 vaccine. In countries where vaccination has begun, limited supplies of vaccine in combination with surges in cases and deaths have overwhelmed health care systems. Further, the emergence of more transmissible SARS-CoV-2 variants has led many to question existing vaccine rollout plans and propose alternative strategies. Some countries and experts have recommended delaying second doses of vaccine to maximize the number of people who receive at least one dose. This raises a critical question: What do we know about vaccines, especially the vaccines for COVID-19, that may allow flexibility in vaccine schedules? This is a complex question and there may not be a straightforward answer. Our knowledge is evolving rapidly as vaccine trials progress and rollout continues, and we must keep in mind that different approaches to vaccine scheduling may be appropriate in different settings. Global inequity in access to COVID-19 vaccines means that these questions around adjusting vaccine intervals are being confronted primarily in wealthier countries as of now; however, these questions may become increasingly relevant around the world as the global vaccination effort continues. No matter how the doses are scheduled, increased manufacturing of safe, effective, quality-assured vaccines is urgently needed. Vaccines prevent millions of deaths worldwide each year. Few public health interventions have had as great an impact on individual and population health. Diseases that once caused a tremendous burden of illness and death are now controlled in many places because of routine immunization programs. Vaccination led to the eradication of smallpox, which saved hundreds of millions of lives, and has led to the near eradication of polio, which previously affected more than 350,000 children each year. Vaccines have also played a defining role in controlling epidemics. During an influenza pandemic, the objective for influenza vaccine distribution is to reduce illness and death and minimize disruption to society and the economy; COVID-19 vaccination programs have similar goals. Although vaccine rollout in a pandemic scenario will always present challenges, in an influenza pandemic, we benefit from years of experience designing, producing and administering influenza vaccines (which typically require only a single dose) and in stockpiling influenza vaccines for a potential pandemic. In contrast, during the COVID-19 pandemic, we are confronting a new infectious disease with concurrent high rates of transmission around the world and rolling out new vaccines that were created, manufactured and authorized in record time, building on many years of research and unprecedented national and global investments. Most currently authorized COVID-19 vaccines, and most COVID-19 vaccines in development, use a two-dose regimen. Two doses may be necessary to provide sufficient strength and duration of immunologic protection, but the need to administer two doses, at a specific time interval, presents additional resource and logistical challenges. As efforts have intensified to rapidly distribute and facilitate uptake of COVID-19 vaccines, there is debate about whether there should be flexibility in the vaccine dosing schedule so that more people can get first doses. In many countries, including the United States, adherence to recommended schedules has been advised, while in others, including the United Kingdom and India, delaying second doses has been advised. It is useful to frame these decisions against a backdrop of how vaccine schedules are typically developed, how COVID-19 vaccine schedules have been developed, local epidemiology, vaccine type, and the evidence we may have to support flexibility within the recommended schedules. How are vaccine regimens, including the number of doses and multidose schedules, typically determined? Why are multiple doses of some vaccines needed? Similar to many COVID-19 vaccines, almost all routine immunizations recommended by the Advisory Committee on Immunization Practices (ACIP) and the World Health Organization (WHO) require multiple doses administered at predetermined intervals. There are different reasons why multiple doses may be needed. For many vaccines, such as hepatitis B vaccine, multiple doses are needed to produce long-lasting immune protection. In this case, the first dose “primes” the immune system by provoking an initial response to the antigens contained in the vaccine. Subsequent doses “boost” the immune response by activating immune cells created after the first dose to produce stronger, longer-lasting protection. For some vaccines, such as the measles, mumps and rubella (MMR) vaccine, just one dose can produce long-lasting protection, but multiple doses are recommended because a small proportion of people do not respond to the first dose or because protection is longer-lasting and more complete after a second dose. (Adding a second dose of the measles vaccine was critical in controlling the measles resurgence in the United States in the early 1990s, and continues to be an essential strategy for the control of measles, which still kills 100,000 children or more each year, globally.) In contrast, for influenza, the genetic code of the circulating virus is different from year to year and vaccine-induced immunity may wane after just a few months. For these reasons, CDC recommends that people get an influenza vaccine annually. For some COVID-19 vaccines, an important reason for multiple doses is to increase the strength of antibody response. As shown in Figure 1 below, antibody titers reached levels of around 10 4 after the first dose of the Moderna COVID-19 vaccine. After the second dose administered on day 29, antibody titers increased to 10 6. Figure 1. Shown are data from 34 participants who were stratified according to age: 18 to 55 years of age (15 participants), 56 to 70 years of age (9 participants), and 71 years of age or older (10 participants). All the participants received 100 μg of Moderna COVID-19 vaccine on days 1 and 29, indicated by arrows. The titers shown are the binding to spike receptor–binding domain (RBD) protein (the end- point dilution titer) assessed on enzyme-linked immunosorbent assay (ELISA) on days 1, 15, 29, 36, 43, 57 and 119. Source: New England Journal of Medicine How do scientists determine the recommended number and schedule of vaccine doses? The number and schedule of vaccine doses are determined during vaccine trials. Traditionally, multiple vaccination schedules are investigated during pre-clinical studies in animals and early clinical trials in humans (Figure 2). A primary objective of Phase II trials is to identify an optimal vaccination schedule. To do this, scientists compare the strength, type and duration of immune responses produced by multiple vaccine schedules and assess the safety of each schedule. According to WHO guidance, vaccine trials should evaluate the shortest proposed interval between doses. Testing for the minimum interval between doses is important because there should be time to allow the body to mount a response to the first dose before the second dose is administered. Figure 2. Difference between traditional vaccine development and development using a pandemic paradigm. The pandemic paradigm requires multiple activities to be conducted at financial risk to developers and manufacturers and without knowing whether the vaccine candidate will be safe and effective, including very early manufacturing scale-up to commercial scale before establishment of clinical proof of concept. ID denotes identification. Source: New England Journal of Medicine Which vaccination schedules are tested during trials is often informed by findings from earlier studies of similar types of vaccines. For example, in developing the Oxford University/AstraZeneca (Oxford) vaccine, which uses an adenovirus-vectored platform to deliver SARS-CoV-2 proteins to the immune system, researchers built on previous experience developing a similar type of vaccine for Middle East Respiratory Syndrome coronavirus (MERS-CoV). This previous research showed that a single dose protected non-human primates against disease. Similarly, testing of the Johnson & Johnson adenovirus-vectored COVID-19 vaccine as both a single dose and two-dose regimen was informed by prior experience with candidate adenovirus-vectored vaccines for other diseases. A Zika virus vaccine candidate generated a durable antibody response after a single dose in non-human primates, whereas an HIV vaccine candidate resulted in a 10-fold higher antibody titer after two doses compared to a single dose. What is the potential impact of deviating from the recommended dose schedule? The effect of changing the interval between doses is likely to vary by vaccine, and not all dose schedules have been studied.
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