In the quest to develop vaccines against Covid-19, researchers have overcome challenges that typically require years, if not decades, to beat. After the first experimental shots produced positive results in late-stage trials -- a huge feat in itself -- drug companies and health officials faced another set of obstacles as they prepared to inoculate hundreds of millions of people around the world. A number of uncertainties remain, including how long any vaccine’s protection against the coronavirus will last. Answering those questions will help tell us how much longer face masks, social distancing, mass testing and contact tracing will be needed to fight the pandemic. Among the hurdles ahead:

Getting the Green Light

The U.K. on Dec. 2 became the first Western country to authorize a Covid shot, using fast-track options regulators have laid out. Scientists hope it will be the start of a series of vaccines to be authorized, produced and distributed. The U.K. cleared a vaccine from Pfizer Inc. and BioNTech SE that was about 95% effective in preliminary analyses of trial results announced in November. In the same month, a second vaccine candidate from Moderna Inc. more or less matched that result, and a third, from AstraZeneca Plc, was 70% effective on average. Months earlier, China and Russia had begun inoculating selected groups, such as medical workers and military personnel, with domestically developed vaccines before they had undergone full testing.

Production Hurdles

The more vaccine designs that prove safe and effective, the better, because of the challenges of mass-producing enough shots to meet global demand. Factories were scaling up manufacturing of special glass vials, needles and other necessary equipment in 2020, and even doses of some vaccines that were still being tested. Supplies will be limited at first: Pfizer said it expected to have doses for as many as 25 million people by the end of the year, and for 650 million in 2021.

Transport and Storage

By one estimate, airlifting vaccines to protect the world’s population would require about 8,000 cargo planes. Complicating matters, some of the vaccines must be kept frozen at temperatures as low as minus 70 degrees Celsius (minus 94 degrees Fahrenheit). Gavi, a nonprofit dedicated to increasing access to immunization in poorer countries, aimed to have 65,000 vaccine refrigerators in place in developing nations by the end of 2020. Most of the Covid vaccine front-runners, including the first three to demonstrate efficacy, depend on two shots. Johnson & Johnson’s may work after just one.

Vaccine Nationalism

Who gets the vaccine first will depend largely on deals that governments have made with drug companies. The U.S., the European Union and the U.K. placed advance orders for billions of doses of vaccine candidates in 2020, unsure which ones would get the nod. Some countries, notably the U.S., committed significant funds to back research, development and manufacturing, racing to protect their own citizens. Public health specialists warn that so-called vaccine nationalism could prolong the pandemic. The most efficient way to allocate shots would be to vaccinate people at high risk of infection and the most vulnerable, such as health-care workers and the elderly, around the world before moving on to entire populations.

Unresolved Questions

A small number of Covid patients have fallen ill twice, raising concerns about waning immunity for a vaccine. Only some viruses, such as the one that causes measles, provide their victims with something close to lifelong immunity after recovery. As for Covid, early studies showed that levels of virus-fighting antibodies in patients who survived a severe case remained high for several months. However, research has also shown that antibody levels decline over time, especially for mild cases. Other questions include how well different vaccines work in the elderly and other high-risk groups, as well as safety. People who receive a vaccine will need to be monitored for months. The answers will help determine how much longer face masks will be de rigueur.

Vaccine Hesitancy

A vaccine rollout can stop a virus from circulating by contributing to so-called herd immunity. But not everyone will embrace the shots. The persistent myth that childhood vaccines pose significant risks has undercut confidence in immunization in many countries. The fast pace at which Covid shots have been developed and concerns about political interference haven’t helped. Polls in the U.S. and Germany in September found only about half of people said they were likely to get a Covid vaccine if it were available. The figure was 68% across seven European countries. Herd immunity comes when enough people in a community have either been infected or vaccinated, and are thus immune. For Covid, the percentage is estimated to range from 55% to 82%.

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