Read the following passage and mark the letter A, B, C or D on your answer sheet to indicate the best answer to each of the following questions from 23 to 30.

Vaccination is one of the most effective public health interventions in human history. The concept of vaccination dates back to the 18th century, when British physician Edward Jenner developed a smallpox vaccine using cowpox material. Jenner’s work paved the way for modern immunology, and his discovery eventually led to the global eradication of smallpox in 1980. Smallpox remains the only human disease to be completely eradicated through vaccination.

Vaccines work by stimulating the immune system to recognize and fight specific pathogens, such as bacteria or viruses. By exposing the body to a harmless part of a pathogen, vaccines "train" the immune system to respond quickly if exposed to the actual disease in the future. Vaccination has led to dramatic reductions in diseases like polio, measles, and tetanus, saving millions of lives worldwide.

Over the years, there has been some controversy surrounding vaccines, largely due to misinformation and misconceptions. Some individuals believe vaccines are unsafe or ineffective, despite extensive research proving otherwise. The anti-vaccine movement has led to a resurgence of preventable diseases, which were once nearly eliminated. Health officials stress the importance of reliable information and public awareness to address these.

Today, vaccination remains an essential tool for disease prevention, especially in the context of global health crises like the COVID-19 pandemic. Scientists are working to develop new vaccines that are more effective, as well as vaccines that do not require refrigeration, making them easier to distribute in remote regions. As the world continues to face new health threats, vaccines will play a crucial role in safeguarding public health.

The word "harmless" in paragraph 2 is OPPOSITE in meaning to _______.