Dear Doctors: I’ve heard that a lot of people don’t get enough vitamin D in their diet. What about getting it from the sun? Is that too risky? Also, what happens if you don’t get enough?

Dear Reader: Vitamin D is a fat-soluble nutrient that our bodies require in order to absorb calcium from the intestines. That calcium is then used to harden the tissues of the skeleton, a process known as mineralization. Vitamin D is also crucial to bone growth, plays a role in managing inflammation, helps prevent involuntary muscle spasms and aids in the regulation of blood phosphorus levels.

Not getting enough vitamin D makes it difficult to maintain adequate levels of calcium and phosphorus, which can cause bones to gradually become thin and brittle. Together with calcium, vitamin D also helps protect older adults from osteoporosis. In children, a lack of the vitamin can lead to rickets, a disease that interferes with mineralization. Rickets is a sometimes painful disease, marked by poor skeletal formation and soft, weak bones that can become malformed.

When our skin is exposed to sunlight, our bodies become natural vitamin D factories. The energy in ultraviolet rays triggers a complex chemical reaction that, along with help from the liver, kidneys and certain cellular structures, results in the formation of vitamin D. The nutrient is also naturally available in fatty fish, including tuna, mackerel, salmon and sardines, and in smaller amounts in certain foods such as beef liver, cheeses, egg yolks and some mushrooms. However, we don’t eat these foods in large enough quantities to satisfy our body’s ongoing need for vitamin D. To help make up for a potential deficit, a range of prepared foods are fortified with the nutrient. These include orange juice, breakfast cereals and dairy products, as well as some soy, rice and noodle products.

The amount of vitamin D that you need to maintain optimal bone health depends on your age. Infants up to 1 year old should get 400 international units (IU) per day. Children, teens and all adults up to the age of 70 are advised to get 600 IU. After age 70, when our bodies become less efficient, the recommendation increases to 800 IU per day.

It’s rare, but possible, to get too much vitamin D. The addition of the nutrient to prepared foods is carefully regulated, and our bodies naturally limit the amount they manufacture from sun exposure, so you’re not going to run into problems there. If you’re going to rely on vitamins or supplements for your vitamin D intake, be sure to check with your health care provider.

Getting enough vitamin D from the sun depends on location, weather and skin type. Also, sunscreen limits how much a person can produce. For those with light skin, 15 to 30 minutes of full sunlight on bare arms, legs or torso a total of two to three times per week is adequate. Due to the protective effects of melanin, darker skin requires longer exposure. People with any kind of skin cancer history or risk should rely on diet and supplements to get their daily allowance.

(Send your questions to askthedoctors@mednet.ucla.edu. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctors: Can you explain herd immunity? We have it with diseases like smallpox and polio, but when it comes to the coronavirus, everyone keeps saying it’s a bad idea. Why wouldn’t it work for COVID-19?

Dear Reader: It’s true that we’re hearing a lot about herd immunity, which is also sometimes known as community immunity, from a variety of sources these days. There’s quite a bit of conjecture, and even misinformation, in some of these discussions, so we’re glad you’ve broached the topic.

Herd immunity refers to the point at which a large enough portion of a population becomes immune to an infectious disease that the illness can no longer be easily spread. This offers protection to the population at large, including to the minority who are not themselves yet immune to the infectious agent.

And you’re correct that we have herd immunity to thank for the eradication of smallpox and the extremely low incidence of polio throughout the world. The reason for this is the success of widespread vaccination efforts, which helped to make the vast majority of people immune to what had previously been serious health threats. In fact, thanks to worldwide smallpox vaccination programs, that disease was declared eradicated in 1980.

Researchers estimate that at least 60% to 70% of the population will have to become immune to the novel coronavirus in order to achieve herd immunity. Unfortunately, we don’t yet have a vaccine to achieve that. That means the only path to herd immunity at this time is for a significant portion of the populace to become infected. Based on the current U.S. population, we’re talking about almost 200 million adults and children becoming infected with the novel coronavirus in order to achieve herd immunity.

As we write this, the total number of novel coronavirus infections in the U.S. has just passed 6.5 million, far short of the 200 million needed for herd immunity. The disease has already caused more than 200,000 deaths since the start of the year, and it has left many survivors with serious and lingering health conditions. So you can see that, while achieving herd immunity through natural infection theoretically is possible, it would come at an unbearable price.

It’s true that the health risks of COVID-19 may recede as we continue to learn about the disease and, hopefully, develop new and more effective treatments. That would make becoming infected with the virus less dangerous. However, another important question remains unanswered. That is, how long do people who have recovered from COVID-19 remain immune? Unfortunately, we just don’t know yet.

All of which brings us back to the discussion we’ve been having for most of this past year. Until there’s a safe and effective vaccine, we need to do our best to slow transmission of the virus. Wear a cloth face covering in public, avoid large gatherings, maintain physical distancing, wash your hands or use a hand sanitizer, avoid touching your face, and disinfect high-touch surfaces. These are challenging times, so please, remember to be kind to one another.

(Send your questions to askthedoctors@mednet.ucla.edu. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctors: I'm 23 years old and hardly ever get sick, not even a cold. I usually skip getting a flu shot, but my dad keeps telling my brother and me that it's really important to get one this year. Can you explain why? Is it going to protect us against the coronavirus?

Dear Reader: Kudos to your dad for spreading the word about flu shots. Although the annual flu shot won't safeguard against the novel coronavirus that causes COVID-19, it will add an important measure of protection during a particularly perilous flu season. As many of you already know, there can be a great deal of overlap in symptoms of the flu and COVID-19. Both are respiratory illnesses that affect the lungs and interfere with the ability to breathe. Symptoms in each can include fever, chills, body aches, shortness of breath, sore throat, headache, cough, chest congestion, difficulty breathing and fatigue. Both the flu and COVID-19 can lead to pneumonia, hospitalization and even death. During last year's flu season, 490,600 people were hospitalized, and 34,200 people died. And even when it's not life-threatening, the flu guarantees a week or two of misery.

As we said earlier, a flu shot won't protect against the novel coronavirus. However, it does reduce the risk of becoming infected with the influenza virus. Some people do get the flu despite having had the vaccine. However, they often have milder symptoms and shorter illnesses than those who go unvaccinated. And this year, with the spread of COVID-19 not yet under control, it's important to do everything we can to make sure medical resources are available for those who are the most seriously ill. It's quite possible we'll see another surge of COVID-19 this winter. With health care workers and facilities already overburdened, we should do everything we can to lessen the strain.

The Centers for Disease Control and Prevention recommend that everyone 6 months of age and older be vaccinated against the flu each year. Inactivated influenza vaccines are approved for children as young as 6 months. That's important because children under the age of 5, and particularly those younger than 2, are at high risk of developing serious complications when they become ill with the flu. That includes ear infections, dehydration and pneumonia. Your child's health care provider will advise you on the appropriate vaccine for your child. Most people get a standard flu shot. Adults 65 years and older, whose immune systems have slowed down, are urged to get the high-dose vaccine formulated specifically for senior citizens. For people with an egg allergy, there is an egg-free version of the flu vaccine that may be appropriate.

The good news is that flu shots are already widely available. They're free with most types of insurance and are often available at discounted rates at flu shot clinics. Free flu shots are also available through community organizations and public health departments. For flu shot locations in your area, visit vaccinefinder.org.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10880 Wilshire Blvd., Suite 1450, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)