THE SHORTAGE of DPT vaccine comes down to a question of product liability. DPT stands for diphtheria, tetanus and pertussis, or whooping cough -- three ugly diseases especially dangerous to small children. The vaccine is highly effective, but the whooping cough component carries a risk of its own. In some children it causes a severe reaction that can result in brain damage or even death. Lawsuits over these reactions have risen rapidly, and manufacturers have been dropping out of the vaccine business.

Spot shortages began to appear here and there last August. They will get worse in the months ahead, and last week the federal Centers for Disease Control recommended drastic conservation by postponing each child's normal booster shots. Rep. Henry A. Waxman, chairman of the House health subcommittee, called a hearing to ask exactly how these shortages arose.

A year ago, three companies sold DPT vaccine. In June two of them, Wyeth Laboratories and Connaught Laboratories, said that they would stop distributing it except to fulfill existing contracts. Both cited the exposure to liability suits. That left only one distributor, Lederle Laboratories. Wyeth continues to make the vaccine but provides it only to Lederle, which sells it and bears the liability. Then last month Lederle ran into production trouble with two batches, and a supply that had been barely sufficient suddenly fell short.

As the number of companies has dropped, the price of DPT has risen like a rocket. It sold for 10 to 12 cents a dose in 1982. Currently the price is $2.80 a dose. Mr. Waxman asked Robert B. Johnson, the president of Lederle, how much of the increase is owed to the risk of litigation. About 30 to 40 percent of it, Mr. Johnson replied. In other words, product liability suits account for a large part of the increase but less than half.

Lederle is entitled to great credit for meeting an urgent public responsibility to continue distributing a product essential to children's health. But it also invites the question whether it is not using this product, in which it now has a monopoly, to subsidize the development of other products in more competitive markets.

The next Congress is going to take up legislation to guarantee the vaccine supply. The industry wants a public system of compensation for victims, relieving the producers of liability for the vaccine's inherent risks. But there's a counterargument that Mr. Waxman cited. If the producers have a guaranteed market and no product liability, will there be any pressure to develop safer vaccines? The point is controversial, but there is some reason to think that whooping cough vaccine can be refined to reduce the numbers of severe reactions. The challenge to Congress and the drug industry is not only to maintain the present level of protection for children, but to keep improving it.