Once vaccine doses are distributed in the millions to a population, we can expect to see cases of COVID-19 dropping in those populations within a few months. Following a decrease in case numbers, we can expect decreases in hospitalization rates (the number of hospitalizations per 100,000 individuals), and then decreases in mortality rates (the number of deaths due to COVID-19 in a population). Importantly, we do not yet know how much we'll see these case numbers drop following the first vaccinations, because we don't yet know how effective the vaccines are outside clinical trials. The decrease could be minuscule or massive — and there's no way of knowing until the vaccines are distributed. The data we have on the most promising vaccines reflect vaccine efficacy, which is different than effectiveness and shows us how well a vaccine works to prevent a particular disease in a controlled, research environment. The data currently shows 95% efficacy for the Pfizer vaccine and 94.5% efficacy for the Moderna vaccine, with efficacy to be announced soon by AstraZeneca. We will not have data on vaccine effectiveness until a vaccine is made available to large populations outside of clinical trials. Given that U.K. Medicines & Healthcare products Regulatory Agency (MHRA) authorized the supply of Pfizer and BioNTech’s COVID-19 mRNA vaccine on December 2, we will likely have data on the Pfizer vaccine's effectiveness first. Once we have information on effectiveness, we’ll have a better sense of how the vaccines will impact metrics such as case numbers, test-positive rates, hospitalization rates, mortality rates, and level of disease severity. If vaccine distribution begins in early-mid-December (the Pfizer vaccine is set to begin being distributed in the U.K. on December 8), by mid-March to the highest risk individuals a population may begin to see declining case numbers. Depending on the rate at which a population is vaccinated, and particularly when distribution of the vaccine moves from highest risk individuals to the broader public, a December distribution start date to high-risk individuals followed quickly (within one month or so) by the general public could potentially vaccinate approximately 70% of that population by mid-late summer (within 6-8 months). This level of vaccination in a population would mean the population reaches herd immunity, triggering significant decreases in COVID-19 metrics such as case numbers, hospitalization rates, and mortality rates. However, if the general public in a population does not have access to vaccinations until later than this timeline (for example, April - June), herd immunity may not be reached until the fall or end of 2021 (6-8 months following). It is important to note that these types of timelines are estimates and based on an assumption that mass vaccination production and delivery is efficient. It also typically takes a few weeks for the body to build immunity after vaccination, so as a result, it is still possible for someone to contract COVID-19 just after they receive a vaccine, influencing case numbers. Case numbers, hospitalization rates, and mortality rates following vaccination in a country will depend on a variety of factors. First, different populations within countries suffer from COVID-19 morbidity and mortality differently. Black Americans, for example, have 2.6x higher case numbers, 4.7x higher hospitalization rates, and 2.1x higher mortality rates than white Americans. This will impact how much case numbers, hospitalizations and deaths fall in those populations after the first doses of vaccines get distributed to Americans. The distribution of vaccines is also not guaranteed to be equitable and there are varying degrees of willingness to even take a vaccine in some populations. Finally, some communities within a country may continue to practice recommended preventative guidelines (i.e. masks, social distancing) after the first rounds of vaccination, while others may not. Current projected distribution plans across the two most promising vaccines is for the vaccine to first go to emergency department clinicians, outpatient clinicians, testers at symptomatic sites, other high-risk health care workers, immunocompromised individuals, EMTs, and potentially essential federal employees, followed by the rest of the general population. However, in the case of Pfizer, they are permitting the regions, countries, and states, distributing the vaccines to determine the distribution plans. For instance, in the U.S., distribution plans are by state. In the United States, vaccines are currently intended to be allocated to all 50 states and eight territories, in addition to six major metropolitan areas. Pfizer, which has filed for emergency use authorization (EUA) in the U.S. and is the closest to potential approval, is prioritizing this approach over a plan that would prioritize the hardest-hit areas of the country, which they’ve said was decided due to the rapid wide spread of the virus. This plan will have impacts for the case rates and other metrics, as equal distribution will lead to different outcomes than targeted distribution. Other barriers to vaccine access include, among others, continual access to health care, particularly given that both the Pfizer and Moderna vaccine require two doses; cost (the vaccines are free but vaccination providers will be able to charge an administration fee); and potential supply chain challenges.