How effective is tetanus-diphtheria toxoid (td)?

93-99%+. Drug company data submitted for review show the vaccine produces a 4x or better increase in protective response in 93-99% of the study group. Researchers consider the 4x boost evidence of protection. Given the decline of tetanus to 40 or less per year in the US it appears something is going well.

Related Questions

When adolescents and adults are scheduled for their routine tetanus and diphtheria booster, should they get vaccinated with td or tdap?

Tdap, Usually. Unless there is a specific reason not to vaccinate against pertussis (whooping cough) such as a previous allergic reaction, tdap is preferred. Currently, people 65 and older can receive a dose of tdap, with boosters of td every 10 years.
Depends. Pertussis is making a comeback and the adults are the no-longer-immune reservoir. So if no tdap in a long time, get tdap. If had one w/in 15yrs, then td ok. No harm in going w/tdap and may help protect the "herd.".
Should be Tdap. We have experience a big surge in cases of pertussis (whooping cough) in recent years. We have also realized the immunity provided by acellular pertussis vaccine wears off in 5 years. Adolescents and adults including pregnant mothers certainly need to get tdap.