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.)

Dear Doctors: I keep hearing different things about at what age and how often a woman should be getting a mammogram. Can you please go over the latest advice?

Dear Reader: It’s true that screening guidelines regarding mammography have changed in recent years. These shifts arise when ongoing research, and the collection of data, suggest a new screening approach that is safer for the patient, or one that offers a better outcome. Various organizations, such as the American Cancer Society, the U.S. Preventive Services Task Force and the American College of Physicians, have slightly different guidelines. To avoid confusion, we’ll focus on the newest advice from the American Cancer Society, released in 2015, and then talk about the specific reasons these guidelines changed.

Women between the ages of 40 and 44 years should have the choice to begin having annual mammograms. Those who wish to do so should speak with their health care provider about the potential benefits and risks. For women between the ages of 45 and 54, the American Cancer Society recommends getting a mammogram each year. Starting at age 55, the organization recommends switching to having a mammogram every other year. However, women can still make the choice to continue annual yearly screenings. Women who are 55 and older should continue to have biennial breast cancer screenings if they are in good health and are expected to live another decade or more.

It’s important to note that these guidelines are for women of average risk. This means they don’t have a family history of breast cancer and have not tested positive for certain genes that are associated with elevated risk for breast cancer. Women who are considered to be at high risk for breast cancer are advised to get a mammogram and an MRI each year. Women with dense breasts, which can affect the accuracy of a mammogram, should discuss the potential efficacy of screening with their health care provider. And don’t forget, adult women of all ages are urged to perform a breast self-exam at least once each month, and to also have a clinical breast exam performed by a health care professional every year.

Although very rare, breast cancer can occur in men. It often presents as a firm lump beneath the nipple. Men with this symptom should see their doctor.

Changes to breast cancer screening guidelines were adopted because of a growing body of evidence that the disease occurs less often in women in their early 40s. Because their breasts are denser, younger women are at increased risk of receiving a false positive result in a mammogram. This would not only cause the woman unnecessary anxiety but would also lead to a range of follow-up procedures. Another concern was over-diagnosis -- that is, the discovery of a cancer that didn’t pose an immediate risk but would then lead to surgery, such as a lumpectomy, and possible follow-up treatments, such as radiation or hormone therapy. The risks of early annual screenings were therefore seen to outweigh the benefits. Ultimately, though, decisions about screening rest with the patient and her health care provider.

(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.)